Distinctive rough seafl oor topography Seafl oor geomorphic manifestations of gas venting and shallow subbottom gas hydrate occurrences

C.K. Paull1, D.W. Caress1, H. Thomas1, E. Lundsten1, K. Anderson1, R. Gwiazda1, M. Riedel2, M. McGann3, and J.C. Herguera4 1Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-9644, USA 2Natural Resources Canada, Geological Survey of Canada - Pacifi c, 9860 West Saanich Road, Sidney, British Columbia V8L4B2, Canada 3U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA 4Centro de Investigacion Cientifi ca y de Education Superior de Ensenada, Carretera Ensenada-Tijuana No. 3918, Zona Playitas, C.P. 22860, Ensenada, B.C., Mexico

ABSTRACT of slow seepage and shows the impact of gas et al., 2008; Jones, et al., 2010). These areas venting and gas hydrate development on the became targets for human occupied vehicle High-resolution multibeam bathymetry seafl oor morphology. (HOV) and remotely operated vehicle (ROV) data collected with an autonomous under- dives to conduct detailed observations and sam- water vehicle (AUV) complemented by com- INTRODUCTION pling programs. Water-column acoustic anoma- pressed high-intensity radar pulse (Chirp) lies have helped to identify other sites (e.g., profi les and remotely operated vehicle (ROV) Here, we report on seafl oor morphologies Merewether et al., 1985; Gardner et al., 2009). observations and sediment sampling reveal a observed where fl uids are venting along the The best grid resolution of ship-mounted distinctive rough topography associated with Pacifi c margin of North America. Areas where multibeam bathymetric data is ~10 m (e.g., seafl oor gas venting and/or near-subsurface hydrocarbon-bearing fl uids are seeping onto Hughes Clarke et al., 1998). While this level of gas hydrate accumulations. The surveys pro- the seafl oor are among the most dynamic deep- resolution nicely reveals the general shape of the vide 1 m bathymetric grids of deep-water gas sea environments. Chemosynthetic biological seafl oor, an appreciable gap exists between the venting sites along the best-known gas vent- communities surround these sites, supported morphologies that are resolvable in surface ship ing areas along the Pacifi c margin of North by energy from hydrocarbon oxidization (e.g., multibeam data and what can be observed using America, which is an unprecedented level of Sibuet and Olu, 1998; Levin, 2005). Diagenetic the fi eld of view provided by HOVs and ROVs. resolution. Patches of conspicuously rough reactions are enhanced in seep environments, Recently, it has become possible to use seafl oor that are tens of meters to hundreds notably those that result in the precipitation of autonomous underwater vehicles (AUV) to map of meters across and occur on larger sea- methane-derived carbonates (e.g., Ritger et al., selected areas of the seafl oor at 1 m grid resolu- fl oor topographic highs characterize seepage 1987; Kulm and Suess, 1990; Paull et al., 1992; tion. The AUV surveys presented here provide areas. Some patches are composed of mul- Bohrmann et al., 1998; Peckmann et al., 2001). a resolution that bridges the gap between visual tiple depressions that range from 1 to 100 m Gas hydrate formation and decomposition occur observations and the best resolution obtain- in diameter and are commonly up to 10 m within the near-seafl oor sediments, altering sea- able from surface ship mapping (≥10 m grids). deeper than the adjacent seafl oor. Elevated fl oor sediment properties (Kvenvolden, 1999; Because these surveys were collected with the mounds with relief of >10 m and fractured Sloan and Koh, 2008). As a consequence, sea- same vehicle, meaningful comparisons can be surfaces suggest that seafl oor expansion also fl oor seepage areas have become a focus of the made between sites, and the recurring charac- occurs. Ground truth observations show research community. Seafl oor seepage sites also teristics of seafl oor fl uid venting sites can be that these areas contain broken pavements pose special geohazard issues, in part because of delineated for the fi rst time. of methane-derived authigenic carbonates the potential for unstable seafl oor conditions and with intervening topographic lows. Pat- the possible existence of overpressured gas in METHODS terns seen in Chirp profi les, ROV observa- the near subsurface (Chiocci et al., 2011). Thus, tions, and core data suggest that the rough the hydrocarbon industry avoids installing sea- AUV Surveys topography is produced by a combination fl oor structures near seeps (Hough et al., 2011). of diagenetic alteration, focused erosion, and Most of the known seafl oor seepage sites Surveys were conducted at seven areas infl ation of the seafl oor. This characteristic were initially identifi ed by regional side-scan (Fig. 1; Table 1) where gas venting occurs texture allows previously unknown gas vent- sonar and surface vessel multibeam bathymetry using an AUV developed at the Monterey Bay ing areas to be identifi ed within these sur- surveys, because exposures of methane-derived Aquarium Research Institute (MBARI) specifi - veys. A conceptual model for the evolution of carbonates and chemosynthetic biological com- cally for seafl oor mapping (Caress et al., 2008). these features suggests that these morpholo- munities result in high seafl oor refl ectivity (e.g., The vehicle carried a Reson 7100, 200 kHz gies develop slowly over protracted periods Carson et al., 1994; Suess et al., 2001; Naudts multibeam sonar, and an Edgetech 2 to 16 kHz

Geosphere; April 2015; v. 11; no. 2; p. 491–513; doi:10.1130/GES01012.1; 16 fi gures; 3 tables. Received 19 December 2013 ♦ Revision received 10 October 2014 ♦ Accepted 25 January 2015 ♦ Published online 27 February 2015

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ples obtained in these dives and from the Inter- Neptune BC CA national Ocean Discovery Program (IODP) core A Barkley Canyon repository (Table 2) were processed in a simi- lar fashion. WA To determine sediment ages, ~25 cm3 sedi- ment samples from selected vibracores were B wet-sieved through a 63 µm screen. Tests of ~1000 planktonic (mixed species) or benthic Hydrate Ridge foraminifera were handpicked from the >63 μm Mexico OR NE Guaymas fraction. Where possible, two samples from vibracores were measured to constrain sediment SW Guaymas accumulation rates. Radiocarbon measurements were made at National Ocean Sciences Accel- erator Mass Spectrometry (NOSAMS) facility Eel Slump CA at the Woods Hole Oceanographic Institution (Table 2). A B RESULTS Figure 1. (A–B) Maps showing locations (red squares) where detailed surveys conducted with Monterey Bay Aquarium Research Institute’s (MBARI) mapping autonomous under- The seafl oor morphology of areas with fl uid water vehicle (AUV) reveal a distinctive recurring morphology (Neptune, Barkley Canyon, seepage is presented here as bathymetric grids Hydrate Ridge, and Eel Slump in A; and Guaymas Basin in B). Inset map (upper right) of 1 m resolution. An overview of previously shows location of parts A and B. All these areas are known to be associated with seafl oor published evidence and/or new ROV observa- fl uid seepage, and the pressure and temperature conditions at the seafl oor are within the tions documenting the occurrence of gas vent- gas hydrate phase stability zone. Location of previously published image from the Santa ing in the survey areas is also presented. Sites Monica Basin shown in Figure 16B is indicated with a triangle. CA—California; OR— are presented from north to south (Fig. 1). Oregon; WA—Washington; BC—British Columbia. Neptune Transect

Chirp subbottom profi ler until the end of 2011, column with only INS navigation available until The existence of gas hydrate along the Casca- when it was replaced with an Edgetech 1 to 6 the seafl oor was within ~100 m. Data processing dia margin is well documented (e.g., Hyndman kHz Chirp subbottom profi ler. The AUV was was done using MB-system (Caress and and Davis, 1992; Westbrook et al., 1994; Hynd- preprogrammed to proceed to >200 waypoints Chayes, 1996; Caress et al., 2008). Subbottom man et al., 2001; Novosel et al., 2005; Riedel during each dive. Missions lasted up to 18 h depths to refl ectors imaged in the Chirp profi les et al., 2002, 2005, 2006a). Three contiguous and were designed for the vehicle to run at a are reported in meters below seafl oor (mbsf) AUV dives mapped the surface of an elongate speed of 3 knots while maintaining an altitude assuming a sound velocity of 750 m per second plateau in the vicinity of the Bullseye Vent node of 50 m off the seafl oor. Track lines were spaced two-way traveltime. on the Neptune cable (Fig. 2). Five subareas of ~150 m apart. In this mode, the AUV obtains distinctive rough topography identifi ed within overlapping multibeam bathymetric coverage at ROV Observations, Sampling, and these three dives are discussed here: Bullseye a vertical resolution of 0.15 m and a horizontal Sample Processing Vent, Bubbly Gulch, Spinnaker Vent, and two footprint of 0.7 m, and Chirp seismic-refl ection ridge crests. profi les with a vertical resolution of 0.11 m. Ini- The ROV Doc Ricketts was used to explore tial navigation fi xes were obtained from global the seafl oor associated with distinctive seafl oor Bullseye Vent positioning system coordinates when the AUV morphologies identifi ed in the AUV surveys Bullseye Vent is associated with a distinct was at the sea surface, and subsequently updated (Fig. 1; Table 1). Doc Ricketts dives provided 305-m-long, 65–75-m-wide, NE-SW–oriented with a Kearfott inertial navigation system (INS) ground truth verifi cation through the collection depression, developed on the surface of a broad and a Doppler velocity log (DVL). The AUV of high-defi nition video and samples, including plateau (Figs. 2, 3A, and 4A). The maximum was launched from the mother ship R/V Zephyr lithifi ed substrates, push cores (≤22 cm long), relief from the rim to the fl oor of the depression and spun down to the seafl oor through the water and vibracores (≤170 cm long). Sediment sam- is 6 m (Fig. 3, B–B′). This distinct depression was not recognized in previous bathymetric surveys because it is below the resolution of TABLE 1. AUTONOMOUS UNDERWATER VEHICLE (AUV) AND REMOTELY surface vessel multibeam sonar. The sides of the OPERATED VEHICLE (ROV) DIVES IN EACH SURVEY AREA depression drop off abruptly along ~5-m-high, Area AUV dives DR dive no. ~25°-sloping scarps. Within the fl oor of the Neptune Bullseye Vent July 23, 24, 25, 2009 61 to 69 Neptune Barkley Canyon July 22, 2009 60 depression, there are ~5 roughly circular sub- Northern Hydrate Ridge July 18, 2009 – depressions that coalesce to form the overall Southern Hydrate Ridge July 17, 2009 58 and 59 Eel Slump July 13 and 15, 2011 280 feature. The seafl oor outside the depression is NW Guaymas Basin March 16 and 18, 2012 379, 380, 385 to 389 smooth but gently shoals to form an elevated SW Guaymas Basin March 23, 2012 390 rim that is ~2 m higher than the surrounding Note: DR—ROV Doc Ricketts. seafl oor.

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TABLE 2. SAMPLE IDENTIFICATION, LONGITUDE, LATITUDE, WATER DEPTH, DEPTH BELOW SEAFLOOR, 14C AGE ± ERROR, RESERVOIR-CORRECTED AGE, AND NOSAMS LABORATORY NUMBER Water Depth below Reservoir- Long. Lat. depth seafl oor 14C age corrected age NOSAMS Sample (°W) (°N) (m) (cm) (yr B.P.) (±) (yr B.P.) Lab. No. Bullseye DR60, VC-22, 25–28 cm1 126.853 48.666 1264 21.5 13,750 60 14,960 OS-79514 DR60, VC-22, 169–172 cm1 126.853 48.666 1264 165.5 15,500 75 17,510 OS-79545 DR60, VC-23, 22–25 cm1 126.852 48.667 1266.2 21.5 18,850 45 21,242 OS-82378 DR60, VC-23, 83–86 cm1 126.852 48.667 1266.2 82.5 20,000 65 22,477 OS-82379 DR60, VC-24, 15–19 cm2 126.852 48.667 1266.4 15,650 70 17,710 OS-79543 DR60, VC-24, sec. 2, 52–57 cm1 126.852 48.667 1266.4 164.5 18,800 75 21,150 OS-79542 DR60, VC-25, 15–18 cm1 126.851 48.667 1266.6 10.5 16,550 40 18,667 OS-82380 DR60, VC-25, 63–66 cm1 126.851 48.667 1266.6 58.5 18,800 70 21,150 OS-82381 DR60, VC-26, 28–32 cm1 126.851 48.668 1264 28 16,950 75 19,060 OS-79515 DR60, VC-26, 146–150 cm1 126.851 48.668 1264 146 18,400 65 20,520 OS-79577 DR60, VC-26, 89–93 cm1 126.851 48.668 1264 89 18,150 75 20,250 OS-82388 DR60, VC-27, 10–13 cm1 126.85 48.668 1264.2 9.5 18,100 70 20,200 OS-82389 DR61, VC-27, 145–148 cm1 126.85 48.668 1264.2 146.5 18,500 65 20,740 OS-82390 DR61, VC-31, 17–20 cm1 126.849 48.669 1258.2 18.5 13,450 50 14,370 OS-82448 DR61, VC-31, 172–175 cm1 126.849 48.669 1258.2 173.5 15,350 70 17,330 OS-82449 DR62, VC-32, 26–30 cm1 126.851 48.668 1257.2 25 14,000 50 15,390 OS-82450 DR62, VC-32, 178–182 cm1 126.851 48.668 1257.2 177 15,100 60 17,040 OS-82451 DR62, VC-34, 20–24 cm1 126.852 48.669 1260 19 13,100 60 13,850 OS-82458 DR62, VC-34, 170–174 cm1 126.852 48.669 1260 169 14,250 50 15,980 OS-82317 DR62, VC-36, 30–33 cm1 126.853 48.67 1259.5 27.5 13,150 60 13,900 OS-79578 DR62, VC-36, 167–170 cm1 126.853 48.67 1259.5 165.5 14,200 55 15,860 OS-79579 Bubbly Gulch DR63, VC-37, 67–71 cm1 126.842 48.675 1259.7 61 20,900 75 23,660 OS-79580 DR63, VC-39, sec. 3, 26–30 cm1 126.842 48.674 1254.3 150 15,050 55 17,000 OS-79521 DR63, VC-39, sec. 3, 26–30 cm2 126.842 48.674 1254.3 150 14,950 65 16,930 OS-79522 DR64, VC-41, 40–43 cm1 126.84 48.675 1268.1 31.5 27,600 120 31,100 OS-79523 DR64, VC-41, 89–92 cm4 126.84 48.675 1268.1 80.5 39,400 330 42,290 OS-79524 DR64, VC-41, 138–141 cm3 126.84 48.675 1268.1 129.5 48,400 870 OS-87588 DR64, VC-43, 66–69 cm1 126.84 48.675 1265.3 60.5 33,100 200 36,542 OS-82476 DR69, VC-66, 63–66 cm1 126.841 48.675 1259.5 59.5 20,000 100 22,500 OS-87594 Spinnaker Vent DR65, VC-45, 133–136 cm1 126.903 48.715 1325.3 128.5 12,450 50 13,230 OS-79525 DR65, VC-45, 133–136 cm2 126.903 48.715 1325.3 128.5 12,200 65 12,980 OS-79526 DR65, VC-46, 18–21 cm1 126.902 48.714 1322.5 7.5 8620 45 8738 OS-79527 DR65, VC-46, 112–115 cm1 126.902 48.714 1322.5 101.5 12,850 50 13,590 OS-79528 DR65, VC-48, 34.5–37 cm1 126.902 48.714 1324.7 35.8 14,000 50 15,390 OS-79529 DR65, VC-49, 23–25 cm1 126.902 48.714 1323.6 18.5 12,200 45 12,990 OS-79530 DR65, VC-49, 94–97 cm1 126.902 48.714 1323.6 89.5 12,350 45 13,157 OS-79532 DR67, VC-56, 132–135 cm1 126.9 48.716 1325.6 133.5 14,150 65 15,750 OS-79533 DR67, VC-56, 132–135 cm2 126.9 48.716 1325.6 133.5 14,050 50 15,480 OS-79534 International Ocean Discovery Program (IODP), Site 204, 1249F Hydrate Ridge South 003H, 01W, 5–11 cm1 128.147295 44.5705283 788.5 8 48,500 630 OS-92733 001H, 01W, 65–71 cm3 128.147295 44.5705283 788.5 68 46,700 680 48,130 OS-92761 IODP, Site 204, 1250C Hydrate Ridge South 001H, 01W, 40–46 cm3 125.150296 44.5687883 807 43 47,900 550 49,370 OS-92736 001H, 01W, 2–8 cm3 125.150296 44.5687883 807 5 43,800 810 45,400 OS-92990 IODP, Site 311, 1327C Neptune Plateau 002H, 04W, 5–10 cm1 126.865 48.698 1304.5 1067.5 14,900 70 16,900 OS-87027 003H, 07W, 80–85 cm3 126.865 48.698 1304.5 2542.5 43,800 870 45,410 OS-87028 004H, 04W, 40–45 cm1 126.865 48.698 1304.5 3002.5 49,200 820 OS-87138 001H, 04W, 70–75 cm1 126.865 48.698 1304.5 522.5 13,950 60 15,320 OS-87203 003H, 04W, 5–10 cm1 126.865 48.698 1304.5 2017.5 33,200 220 36,610 OS-87219 IODP, Site 311, 1328B Bullseye Vent 002H, 02W, 1–6 cm1 126.851 48.667 1267.8 553.5 23,000 90 26,290 OS-87213 009H, 02W, 5–10 cm1 126.851 48.667 1267.8 3904.5 50,400 840 OS-87217 008H, 02W, 50–55 cm3 126.851 48.667 1267.8 3002.5 45,500 970 46,980 OS-87591 004P, 01W, 31–36 cm1 126.851 48.667 1267.8 1483.5 40,000 440 43,150 OS-87592 006X, 01W, 100–105 cm1 126.851 48.667 1267.8 1942.5 40,300 370 43,390 OS-87706 Note: These include two sediment samples (Hydrate Ridge) from near the seafloor in Ocean Drilling Program (ODP) Leg 204, Sites 1249 and 1250 (Tréhu et al., 2003), five samples from the IODP Leg 311, Site 1327 (Neptune), with subbottom depths between 522.5 and 3002.5 cm below seafloor (cmbsf), and five samples from IODP Leg 311, Site 1328 (Bullseye Vent), with subbottom depths between 552.5 and 3907.5 cmbsf. Material used for 14C measurements: 1—mixed planktonic foraminifera; 2—planktonic foraminifera Neogloboquadrina pachyderma; 3—mixed benthic foraminifera; 4—mixed planktonic and benthic foraminifera. Ages were calculated using the accepted half-life of 14C of 5568 yr (Stuiver and Polach, 1977). The original measurements were obtained as 14C/12C ratios, corrected for isotope fractionation by normalizing with δ13C by National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS), and were converted to reservoir-corrected ages using the CALIB 6.0.1 program (Stuiver and Reimer, 1993; Hughen et al., 2004; http://calib.qub.ac.uk/calib/calib.html). A reservoir age of 1750 yr was chosen for the benthic foraminiferal samples following Mix et al. (1999). A reservoir age of 800 yr was used for the planktonic foraminiferal samples with radiocarbon ages younger than 12,000 yr (Southon et al., 1990; Kienast and McKay, 2001), and 1100 yr for those older than 12,100 yr (Kovanen and Easterbrook, 2002).

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1340 1360 1410 1292 1174

1380 Water Depth (m) Fig. 7

1400 SV

Fig. 4C 889 1327 RC Fig. 4B

1400 RC 1240

1340

1200 Fig. 3

1340 1260 BG

1300 Fig. 4A 1328 890 BV 1340

Figure 2. Map showing multibeam data collected on three autonomous underwater vehicle (AUV) dives (color scale ranging from 1410 m to 1174 m water depths) covering part of a NW-SE–trending plateau on the continental margin off British Columbia that is crossed by the Neptune submarine cable (Fig. 1). Locations of Bullseye Vent (BV), Bubbly Gulch (BG), Spinnaker Vent (SV), and two ridge crests (RC) are indicated. Patches of the distinctive rough topography are outlined with thin black lines. The locations of the Neptune cable and node are indicated with the purple lines and pyramid. Ocean Drilling Program (ODP) Sites 889 and 890 and International Ocean Discovery Program (IODP) Sites 1327 and 1328 boreholes are also indicated. Locations of Figures 3, 4A–4C, and 7 are indicated with red boxes. Background bathymetry is based on surface vessel multibeam data provided by Debora Kelly (University of Washington).

Observations during ROV dives show that wall (Fig. 5A) appear to be the edges of trun- ever, the continuity of these refl ectors progres- the seafl oor inside the depression is very hum- cated sedimentary beds. Outside the depression, sively diminishes with proximity to the Bullseye mocky, with scattered pieces of carbonate rub- the seafl oor is smooth and lacks rubble. Vent depression, and a seismic blanking zone ble, shells, and a few small clusters of living AUV Chirp profi les resolve distinct subhori- occurs underneath the depression. Although a Vesicomya clams. Slabs of authigenic carbonate zontal refl ectors down to ~50 mbsf across most slight upturn in the refl ectors within 200 m of (~1–4 cm in thickness) sticking out of the scarp of the plateau away from Bullseye Vent. How- the structure indicates a subtle doming centered

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C ′ Bubbly A Gulch D B 1250

B C A

D 1320 E

Bullseye Vent

B′ C′ Vibracores IODP 1328 Piston Cores E D′ ′ Photo Locations

1350 1285 1220 A Water Depth (m)

1290

Bullseye Vent Bubbly Gulch

20 m VE 10x

100 m

Blanking

A A′ Figure 3. Map showing Bullseye Vent and Bubbly Gulch within the Neptune survey area (Fig. 1). Neptune node is indicated by the pyramid. Color scale ranges from 1350 m to 1220 m water depths. Line A–A′ is the track of autonomous underwater vehicle (AUV)–collected Chirp profi le showing the line extending NE-SW across the long axis of Bullseye Vent depression to Bubbly Gulch. Green line encircling Bullseye Vent outlines area of blanking identifi ed in Chirp profi les B–E shown in Figure 6. Note that the area encircled by the green line is considerably larger than the depression. Also note some refl ectors can be traced laterally from near the depression associated with Bullseye Vent to where the associated units crop out on the slope on the side of the ridge in Bubbly Gulch. Locations of remotely operated vehicle (ROV)–collected vibracores (gray circles), International Ocean Discovery Program (IODP) Site 1328B (red circle), and previously collected piston cores (black triangles; Novosel et al., 2005) are indicated. Locations of video images shown in Figure 5A–5D are indicated. VE—vertical exaggeration.

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underneath Bullseye Vent, the refl ector geom- A etry suggests intact strata at one time continued A undisrupted through what is now the blanking zone (Figs. 3, section B–B′, and 6). 1271 1263 1255 Water Depth (m) Bubbly Gulch Persistent water-column acoustic anomalies (Riedel et al., 2010) have been identifi ed above a small WNW-ESE–oriented embayment called Bubbly Gulch, located on the fl ank of the pla- teau, northeast of Bullseye Vent. AUV surveys partially mapped this structure, which extends laterally ~850 m into the adjacent basin (Figs. 2 and 3). The head of Bubbly Gulch consists of an ~120-m-radius amphitheater with 30 m relief and a smooth slope, except for a subtle bench 90 m NE of its headwall. Refl ectors from the B Chirp profi le, which can be traced underneath the width of the plateau, crop out along a subtle terrace rimming the sides of this re-entrant (Fig. 3, B–B′). Observations during ROV dives identifi ed small (≤1 m high, and 5–10 m wide) mounds E (Figs. 5B and 5C) rimming the sidewall of the embayment, at depths between 1270 and 1265 mbsf, coincident with the bench resolved in the AUV bathymetry. The mounds are covered with white patches of bacterial mats (Fig. 5B), which occasionally exhibit 5-cm-wide open cracks in hexagonal grid patterns suggestive of blistering and expansion of the seafl oor surface (Fig. 5C). 1240 1212 1184 Water Depth (m) Vibracore collection on the mounds along this bench elicited vigorous release of gas (Fig. 5D). Spontaneous periodic gas bubbling was also C seen emanating from the cracks. However, no indication of authigenic carbonate, tubeworms, or living Vesicomya clams was seen.

Ridge Crest Sites Patches of rough topography that contrast with the smooth seafl oor surrounding them were identifi ed on the crest of some of the ridges jut- ting out of the plateau. One of them is at the western end of the large ridge on the NE edge of the AUV survey (Fig. 2). This patch of rough topography consists of a series of coalescing depressions ranging in width from 10 to 70 m 1315 1297 1306 and in depth from 1 to 5 m (Fig. 4B). Character- Water Depth (m) istically, the smooth seafl oor turns up at the rim of the patch of rough topography. ROV observa- Figure 4. (A–C) Bathymetry based on 1 m grid derived from autonomous underwater vehicle tions (Fig. 5E) indicate that the seafl oor has a (AUV) data associated with Bullseye Vent and two unnamed ridge crests within the Nep- nearly continuous pavement of carbonate, rising tune survey (Figs. 2 and 3), which show a distinctive rough seafl oor texture. Color scales toward the rim of the depressions, cut by occa- indicate water depth ranges. (A) Bullseye Vent is a depression ~350 m long and 5 m deep sional cracks and open joints, with few scattered with slightly upturned sides. Location of video image shown in Figure 5D is indicated with clamshells and few isolated clusters of living star marked “A.” (B–C) Seafl oor surfaces on the crests of two of the regional ridges are Vesicomya clams. The walls of the depressions marked with a series of round depressions, which vary from 20 to 100 m in diameter and expose the truncated beds of the pavement, have up to 5 m of relief with respect to the surrounding seafl oor. Location of video image which are rough and heavily cemented, produc- shown in Figure 5E is indicated. ing more than 1 m of overhang in places. The fl oors of the depressions have variable amounts

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A B

C D

E F

G H

Figure 5. Video images from remotely operated vehicle (ROV) dives illustrating the seafl oor in areas of distinctive rough topography. Locations of video images A to D are shown in Figure 3. (A) Thin layers of carbonate-cemented horizons exposed on the northern edge of Bullseye Vent depression. (B) Subtle mound on the fl anks of Bubbly Gulch with white patches believed to be bacterial mats on its crest and outlining cracks on its side. (C) Image shows open cracks on the seafl oor within Bubbly Gulch, which form a polygonal pattern. Parts of the ROV’s mechanical arm (left) and an instrument probe that was to be deployed on this dive (right) are in the foreground. (D) Plume of gas emanating from the hole produced by the collection of an ~1-m-long vibracore into the topographic mound show in part B (appendix video [see text footnote 1]). (E) Slabs of carbonate-cemented strata within the ridge crest depression (Fig. 4B). Note upper surface is smooth, while sides expose irregularly textured carbonates in open cracks. (F) Small mound in the Spinnaker Vent area that has a crater-like depression on its crest and an open crack along its lower fl ank. (G) Image looking into the open crack forming overhanging ledge (shown in part F) where layer of massive gas hydrate is exposed on the side of a mound at Spinnaker Vent. Fish is a snailfi sh (Liparidae, Careproctus sp.). (H) Heavily eroded block of exhumed authigenic carbonate containing open voids and distinctly undercut base from the Spinnaker Vent location shown in Figure 7. Note the high abundance of crabs, numerous snail egg cases (yellow columns), and Vesicomya clam shells scattered at the base of this ~1-m-high face. Field of view is ~3 m in A, B, C, E, F, and H; ~2 m in D; and ~50 cm in G.

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of scattered rubble, occasional larger isolated slabs of carbonate, and some apparently recent sediment fi ll. Angular slabs of lithifi ed mud- stone, presumably from the underlying forma- 20 m VE ×5 tion, are exposed in the deeper depressions. B Another patch with ~12 similar depressions was 100 m also identifi ed on the crest of a smaller ridge B′ (Figs. 2 and 4C). Spinnaker Vent Spinnaker Vent is another site of persistent Blanking water column acoustic anomalies, 6.3 km NW of Bullseye Vent (Figs. 2 and 7). Despite the absence of identifi able features in surface ves- sel multibeam data, 1-m-resolution AUV sur- veys show the presence of an ~500-m-long, ~100-m-wide, NE-SW–oriented patch of dis- tinctive rough topography. The patch is domi- C nated by a 50–70-m-diameter and 2-m-deep C′ depression, with smaller 5–12-m-diameter and 0.5-m-deep depressions to the NE and SW. Two topographic highs or mounds up to 2 m tall also occur on the SW side of the patch (Figs. 5F and 5G). Chirp profi les from a broad region around Spinnaker Vent resolve ~10 m of horizontal Blanking strata around Spinnaker Vent, but in proximity to the rough topography, the refl ectors dome upward. Seismic blanking is present, and ≤2 m section of layered strata is resolved under the area of rough topography (Fig. 7, F–F′). D ROV observations document that the walls ′ of the central depression consists of steep and D occasionally overhanging, 1–2-m-tall scarps built from the truncated edges of rough authi- genic carbonates beds of considerable permea- bility (Fig. 5H). The fl oor of the depression was resistant to coring and contained scattered Blanking carbonate rubble, shell hash, and small beds of living Vesicomya clams, especially at the base of the small scarps on the sides of the depressions. The two ~2-m-high mounds to the SW of the depression exhibited surface cracking and had a small surface crater (Fig. 5F) from which E ′ intermittent gas bubbles emanated. Horizontal E layers of up to 10-cm-thick, solid white mate- rial were exposed in the cracks (Fig. 5G), which were inferred to be gas hydrate because they fl oated away when broken loose. The process of collecting push cores from the fl anks of these Blanking mounds stimulated vigorous gas releases. Age Constraints Along the Neptune plateau, 14C content was measured on 41 foraminifera samples from the Figure 6. NW-SE–oriented autonomous underwater vehicle (AUV)–collected Chirp profi les vibracores and 10 samples from IODP Leg 311, extending across Bullseye Vent. Location of lines is indicated in Figure 3. Extent of under- Sites 1327C and 1328B (Fig. 2; Table 2). All lying zone of seismic blanking that surrounds the topographic depression is illustrated in cores with two or more measurements show Figure 3. VE—vertical exaggeration. increasing age with depth (Fig. 8). The cores from Bubbly Gulch came from the fl ank of the plateau and sampled the truncated edge of units

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F′

1326

1322 H

1324

F & G F

1329 1324 1319 Water Depth (m)

Blanking

10 m VE ~ 5 × F 50 m F′

Figure 7. Image showing detailed bathymetry of the Spinnaker Vent area and a crossing Chirp profi le F–F′. Loca- tion of detailed map is indicated in Figure 2. Color scale ranges from 1329 m to 1319 m water depths. Locations of remotely operated vehicle (ROV)–collected vibracores (red fi lled circles) and video images shown in Figures 5F, 5G, and 5H (stars) are indicated. VE—vertical exaggeration.

that can be traced in Chirp profi les underneath silts and clays (Clague and Ward, 2011) were ceased in this area ca. 13,100 14C yr B.P. Simi- the plateau into the Bullseye Vent depression accumulating from at least 50,000 up to 13,100 larly, the ages of the sediment near the seafl oor at blanking zone (Fig. 3). 14C yr B.P. at rates between 0.4 m/k.y. and 1.1 Spinnaker Vent are ≥12,200 14C yr B.P., with the Sediment accumulation rates calculated from m/k.y. (Fig. 8D). Holocene age sediments are exception of an 8620 14C yr B.P. measurement the age versus depth regressions from Bullseye noticeably absent across the plateau, and the on sediment from within 18 cm of the modern Vent, the fl ank of Bubbly Gulch, and IODP Sites intersection of the seafl oor age versus depth seafl oor. Again, signifi cant accumulations of 1327C and 1328B indicate that glacial marine regressions suggests sediment accumulation Holocene sediment are noticeably absent.

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Outside

Inside

Outside Inside

A B

14C Age (yr) 14C Age (yr)

Outside Bubbly Gulch Inside

IODP 1328

IODP 1327 C D

14C Age (yr) 14C Age (yr)

Figure 8. Plots of the 14C age vs. depth in cores from Bullseye Vent and Spinnaker Vent (Fig. 2; Table 1). (A) 14C age vs. subbottom depth for samples from remotely operated vehicle (ROV)–collected vibracores from Bullseye Vent (with the depth scale being in meters below seafl oor [mbsf]), with colored lines connecting samples from individual cores. Samples from cores taken within Bullseye Vent depression are connected by red lines, while those that came from outside the Bullseye Vent depression are connected by blue lines. (B) Same 14C age vs. depth as in A but with the depth scale being in meters below sea surface (mbss). Dashed line is for the core on the SW side of the depression, which is from greater water depth than the other cores from outside the depression. (C) 14C age vs. subbottom depth for samples from ROV- collected vibracores from Spinnaker Vent. (D) 14C age vs. depth in meters below sea surface (mbss) for samples from Bullseye Vent (blue circles—vibracores from outside the depression; red circles—vibracores from within the depression; black circles—International Ocean Discovery Program [IODP] Site 1328B), green triangles—Bubbly Gulch, and purple squares—IODP 1327C. Lines are linear regression best fi ts to the data from Bullseye Vent (black line), Bubbly Gulch (green line), and IODP 1327C (purple line). Measurements were made on planktonic and benthic foraminifera, except for the outlier sample (identifi ed with a star) in C, which was made on a Vesicomya shell.

The ages of sediment samples from vibracores Barkley Canyon of small topographic mounds on a terrace on the show that the near-surface sediments within the western fl ank of the canyon (e.g., Chapman et al., Bullseye Vent depression are systematically older The occurrence of gas hydrate on the fl ank of 2004; Pohlman et al., 2005; Hester et al., 2007). (≥15,100 14C yr B.P.) than the sediments on its Barkley Canyon was originally discovered by a Bathymetry collected during an AUV survey rim, but outside the depression. The age verses commercial fi shing trawler (Spence et al., 2001; in 2009 shows a line of ~10 circular mounds, depth progression seen in these cores is consis- Fig. 9). Subsequently, ROV dives in ~860 m up to 2 m high and 10 m in diameter, on the tent with the removal of overburden equivalent to water depths showed lenses of pure gas hydrate terrace associated with the gas hydrate occur- the depth of the depression (Figs. 8B and 8D). exposed on the seafl oor in cracks on the fl anks rences (Fig. 9, inset). No internal refl ections

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G′

1022 861 700 Water Depth (m)

G

880 843 806 Water Depth (m)

G

GHOA 40 m VE ~ 5× G′ 200 m

Figure 9. Images show bathymetry of Barkley Canyon based on 1 m grids derived from autonomous underwater vehicle (AUV) data overlain on regional surface vessel multibeam bathymetry data (courtesy of Debora Kelly, University of Washington). Color scale ranges from 1022 m to 700 m water depths. Location of Barkley Canyon is indicated in Figure 1. Red box shows area covered by inset map (lower right) and corresponds with the area where gas hydrates were discovered (Spence et al., 2001). This area contains a series of distinctive topographic highs that are up to 3 m high and up to 20 m in diameter outlined with thin black lines. Location of Chirp profi le G–G′ shown below is indicated. The locations of the Neptune cable and nodes are indicated with the purple lines and pentagonal shapes. Chirp profi le G–G′ extends from the canyon fl oor, where a drape of horizontally layered sediments occurs, across the terrace on the fl ank of the canyon, which lacks layered sediments in the gas hydrate occurrence area (GHOA). VE—vertical exaggeration.

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A B 892A ′ 892D H H

700 800

H′

H 985 785 585 Water Depth (m) 20 m VE 10 × 100 m

Figure 10. Map showing multibeam data (color scale ranging from 985 m to 585 m water depths) collected on an autonomous underwater vehicle (AUV) dive on the crest of northern Hydrate Ridge (see Fig. 1 for location). Patches of the distinctive rough topography are outlined with thin black lines. Locations of Ocean Drilling Program (ODP) Leg 146, Sites 892A and D, are indicated with black fi lled circles. Location of Chirp profi le H–H′ (shown below) is indicated. No subbottom refl ectors occur in Chirp profi le H–H′ underneath areas of rough topography. Blue arrow indicates position of three weak refl ectors observed at the eastern end of the profi le immediately adjacent to the patches of rough topography, which appear to truncate at the seafl oor where the exposed rough topography begins. Red boxes A and B outline areas shown in more detail in Figures 12A and 12B. VE—vertical exaggeration.

are resolved in the Chirp data underneath this (e.g., up to 1400 m long), and by the extent of ~100 m across with ≥35 m of vertical relief (Figs. terrace near where the gas hydrate mounds are relief (commonly 10 m or more) within these 11 and 12C) and occurs within the center of a found. However, up to 50-m-thick sections of patches. One of these patches contains a roughly ~500-m-wide depression in the western patch of fi nely layered sediments occur in places on the circular crater-like feature with a raised rim sur- distinctive rough topography. fl oor and fl anks of the canyon (Fig. 9, G–G′). rounding a 40-m-deep depression that is 300 m Chirp data from Hydrate Ridge are charac- ROV observations confi rm the absence of sedi- in diameter (Figs. 10 and 12A). A ridge extends terized by a strong seafl oor refl ection with only ment drape on the mounds. Although the rock over 600 m to the SW from this crater-like fea- a few wispy sub-bottom refl ectors identifi ed samples from the mounds are mudstones barren ture. This ridge has a trough near its crest inter- during these surveys. These refl ectors outline of microfossils, their fi rm brittle texture suggests preted to be a tension fracture formed during irregular surfaces that are up to 15 mbsf (Fig. these mounds developed within pre-Quaternary infl ation of the seafl oor. 10, section H–H′; Fig. 11, section I–I′) and that strata exposed on the side of the canyon. The AUV data show that the surface of south- consistently pinch out around the areas of rough ern Hydrate Ridge is generally smooth, except for topography. No subbottom refl ectors can be Northern and Southern Hydrate Ridge two contiguous patches of distinctive hummocky identifi ed under the areas of rough topography. topography with maximum diameters of 350 m Foraminifera from 2 to 71 cmbsf at Sites Extensive areas of authigenic carbonates, liv- and 500 m (Figs. 11 and 12C). These areas are 1249 and 1250, which are both within the areas ing chemosynthetic organisms, and gas hydrate ringed with small, apparently erosional scarps, of distinctive rough topography on southern in the near subsurface (e.g., Kulm et al., 1986; and thus the strata exposed within these patches Hydrate Ridge, yielded ages of ≥43,800 14C yr Westbrook et al., 1994; Bohrmann et al., 1998; are stratigraphically lower than the surround- B.P. (Table 2). U/Th measurements on samples Suess et al., 1999, 2001; Tréhu et al., 2003) were ing smooth seafl oor. The fi ne-scale topography from northern Hydrate Ridge indicate that the documented on Hydrate Ridge by HOV and within these circular rough patches is character- timing of cement formation on the seafl oor was ROV dives and Ocean Drilling Program (ODP) ized by small, sometimes circular, ~0.5-m-deep 68.7–71.7 k.y. B.P. (Teichert et al., 2003). and IODP drilling (Figs. 10, 11, and 12). pits, and local highs and lows separated by The surface of northern Hydrate Ridge has ~0.5-m-high ledges, which appear to be irregu- Eel Slump an abundance of distinctive rough topography larly eroded bedding surfaces. Similar shapes in multiple patches along the crest of the ridge also occur at larger scales. The topographic high Water-column plumes were discovered off (Figs. 10 and 12). This site is distinguished from previously called the Pinnacle (Suess et al., 2001; the coast of northern California (Fig. 13; Gard- the others by the large size of individual patches Tréhu et al., 2004; Paull and Ussler, 2008) is ner et al., 2009). Some plumes emanate from

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ing that the slope failure that created the large scarp was not recent. No subbottom refl ectors are traceable underneath the ovoid mound in the center of Eel Slump (Fig. 13, J–J′).

NE Flank of the Guaymas Basin C I After water-column acoustic anomalies were 1249 discovered along the transform margin on the 1250 NE fl ank of the Guaymas Basin, Gulf of Califor- nia (Lonsdale, 1985; Merewether et al., 1985), I′ subsequent investigations in this area showed extensive areas of methane-derived authigenic carbonate, seafl oor chemosynthetic commu- nities, vigorous gas venting, and gas hydrate exposures (Paull et al., 2007). Two AUV dives mapped a ~3.5 km section of a NW-SE–ori- ented ridge with a crest in 1540–1600 m water depths (Fig. 14). The orientation of this ridge

900 is coincident with the trend of the plate margin in this area. The ridge crest is associated with a blanking zone, but strata extending from under- neath the Guaymas Basin thin and/or onlap on the SW side of this ridge (Fig. 14, L–L′). A 930 849.5 769 Water Depth (m) ′ gas vent known as Pinkie’s Vent occurs on the I steeper NE side of this ridge (Fig. 14; Paull et al., 2007). The ~1.4-km-wide basin to the NE of the main ridge has smaller NW-SE–oriented ridges that are up to 1.9 km long and have up to 70 m of relief. Patches of distinctive rough topography con- I 20 m VE 10 × taining circular mounds and depressions occur 100 m along the crests of the main ridge and subsidiary ridges (Figs. 14A and 14B). The largest mound Figure 11. Map showing part of the multibeam data (color scale ranging from 930 m to has 9 m of local elevation within its 130 m diam- 769 m water depths) collected on an autonomous underwater vehicle (AUV) dive on the eter (Fig. 14B), but it occurs on a 750-m-long, crest of southern Hydrate Ridge (see Fig. 1 for location). The area associated with the dis- ~10-m-high swell on the main ridge. The largest tinctive rough topography is outlined with a thin black line. Locations of International Ocean depression is 60 m in diameter and 4 m deep, Drilling Program (IODP) Leg 204, Sites 1249 and 1250, are indicated with fi lled black and is fl anked by elevated topography on both circles. Location of Chirp profi le I–I′ is indicated. Chirp profi le I–I′ (shown below) crosses sides (Fig. 14B). ROV observations confi rm the feature known as the Pinnacle (indicated with a red arrow). The Chirp profi les resolved that the topography consists of irregular, bro- only an occasional wispy refl ector during this survey (blue arrow), but it is consistently ken pavements of methane-derived authigenic truncated at the seafl oor adjacent to where the exposed rough topography begins. Red box carbonates and contains both living and dead C outlines area shown in more detail in Figure 12C. VE—vertical exaggeration. chemosynthetic communities. Chirp data show that the main ridge is com- posed of a wedge of upturned sediments that dip a topographic high that is ~650 m long, 350 m ROV observations show scattered living and to the WSW and thin to the NNE, where they wide, and stands nearly 60 m higher than the dead Vesicomya clams and patches of white bac- are truncated on the fl ank of the ridge (Fig. 14, surrounding seafl oor at 1850 m depth. This top- terial mat on the fl anks and crest of this topo- L–L′), The continuity of the refl ectors is lost ographic high occurs within a 3.5-km-wide slide graphic high. Nearly continuous streams of gas underneath the areas of distinctive rough topog- scar informally known as Eel Slump. bubbles were observed emanating from a few raphy (Fig. 14, M–M′). The surface of this topographic high is char- places on the crest of this topographic feature. acterized by a hummocky topography consisting Analysis of this gas indicates that it is primarily SW Flank of the Guaymas Basin of small, approximately circular, ~0.5-m-deep methane, mixed with other thermogenic hydro- pits, and local highs and lows separated by carbon gases (Gwiazda et al., 2011). Small A previously unexplored area, 3.6 km long ~0.5-m-high ledges that could have been formed drops of oil were also released from the near- and 3.2 km wide, on the SW side of Guaymas by irregular erosion of bedding surfaces (Fig. surface sediment when a push core was taken. Basin was mapped, revealing two elongated 13). A semicircular crater-like depression that is Chirp profi les show that a drape of layered topographic ridges that straddle what is believed ~80 m across and more than 10 m deep occurs sediment that is at least 40 m thick covers the to be the main trace of the transform fault (Fig. on the NW fl ank of this feature. bottom and sidewalls of Eel Slump, suggest- 15). A distinct NW-SE–oriented, 2.8-km-long

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Figure 12. Maps showing the bathymetry based on 1 m grids derived from autonomous A underwater vehicle (AUV) data of two sites on northern Hydrate Ridge (A and B) and one site from southern Hydrate Ridge (C), which show a distinctive rough seafl oor tex- ture. Locations of A and B are indicated in Figure 10, and location of C is indicated in Figure 11. (A) An ovoid-shaped depression that is ~350 m across and 50 m deep. This is at the end of an E-W–oriented ridge with very distinctly rough topography consist- ing of local depressions or open cracks that 842 806 770 are up to 10 m deeper than the surrounding Water Depth (m) area. (B) Another area with very rough local topography that appears to be elevated from the immediate surrounding seafl oor near Ocean Drilling Program (ODP) Leg 146, B Holes 892A and 892D. (C) The only area of distinctive rough topography from the crest of southern Hydrate Ridge. Red arrow iden- tifi es the Pinnacle, which occurs within the 892A center of an ~500-m-wide depression appar- ently associated with a slide scar. Locations of ODP Leg 204 Sites 1249 and 1250 are indi- cated with fi lled black circles. 892D

700 677.5 655 trough, only ~100 m wide, separates these Water Depth (m) ridges. From this trough, the ridge to the SW rises over 140 m. The ridge on the NW side is smaller, with only 30 m of relief with respect to the trough. Ovoid patches of a distinctive rough topography occur along the crests of both C of these ridges (Fig. 15), which are 50–150 m across and contain 1–5-m-deep depressions and roughly circular topographic highs. ROV observations show that the patches 1249 of distinctive rough topography on both ridge crests are associated with exposures of methane- derived carbonates. Beds of living Vesicomya clams and tubeworms also occur on the crests 1250 of both ridges within the areas of distinctive rough topography. On the larger ridge, living clam beds occur discontinuously along a 1-km- 875 787.5 700 long stretch of the ridge where turbidite beds Water Depth (m) containing sandy horizons outcrop. Open cracks between blocks of carbonates and depressions in the seafl oor that are ~1 m deep were observed in the ovoid patches of high relief on the NW fl ank (Fig. 15, inset). mixtures of circular depressions and local topo- evidence for hydrocarbon seepage includes graphic highs. The distinctive topography occurs water-column acoustic anomalies, widespread DISCUSSION within approximately ovoid patches that range occurrence of methane-derived authigenic car- in size from a few tens of meters to hundreds bonates in the upper layers of the seafl oor, liv- Seafl oor surveys using AUVs provide bathy- of meters long and commonly have between ing and dead chemosynthetic biological com- metric grids with 1 m resolution and show areas 2–10 m of local relief (Figs. 4, 7, and 9–16). munities, occasional direct observations of associated with distinctive rough seafl oor topog- This distinctive rough topography occurs in gas bubbling and of gas hydrate exposures on raphy at multiple sites along the west coast of areas where the near-seafl oor sediments have the seafl oor, samples of gas hydrate recovered North America. The topography is composed of been exposed to methane-rich pore waters. The in boreholes, and blanking zones in seismic

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2005 1562.5 1120 Water Depth (m) Figure 13. Map showing the multibeam data (color scale ranging from 2005 m to 1120 m water depths) collected during an individual autonomous under water vehicle (AUV) dive to map a slump scar on the margin south of Eel Canyon J′ (see Fig. 1 for location). On the J sole of this slide scar, there is an isolated 600-m-long topo- graphic high that is associated with water-column acoustic anomalies. Patches of the dis- tinctive rough topography are outlined with thin black lines. Red box indicates area shown in more detail in the lower left, illustrating the distinc- tive rough seafl oor texture that occurs on the crest of this topo- graphic high. Location of Chirp profi le J–J′ is indicated on map, which crosses a corner of this elevated topographic feature. Chirp profi le J–J′ shows that ′ >20 m of horizontally layered J refl ectors occur on both sides of the mound, but no internal refl ections appear underneath 1885 1839 1793 the mound. VE—vertical exag- Water Depth (m) Blanking geration. 20 m VE 10 x J 100 m

profi les. All the areas with this distinctive topog- sites and associated with increased biological Caldwell et al., 2008). AOM occurs within raphy are at water depths (i.e., pressures) and activity, and sediment rafting due to the inherent marine sediments along the sulfate methane temperatures where methane hydrates are stable buoyancy of gas hydrate. transition zone where the concentration of both at the seafl oor and within the near subsurface methane and sulfate are nearing zero (e.g., (Sloan and Koh, 2008). Authigenic Carbonate Formation Borowski et al., 1999; Ussler and Paull, 2008; Here, we argue that the distinctive seafl oor Bhatnagar et al., 2008). The dominant fi ne- morphologies are produced by a combination ROV observations show that the distinc- grained lithology of these authigenic carbon- of widespread chemical changes, including tive topography is consistently associated with ates indicates they were formed slowly within diagenesis associated with anaerobic methane exposures of methane-derived authigenic car- the host strata as a by-product of AOM (e.g., oxidation, and physical changes. The physical bonate (e.g., Ritger et al., 1987; Paull et al., Ussler and Paull, 2008). changes include seafl oor infl ation and collapse 1992; Sakai et al., 1992; Bohrmann et al., 1998; The addition of authigenic carbonate cement associated with formation and decomposition Stakes et al., 1999; Roberts, 2001). Methane- changes the mechanical properties of hemi- of gas hydrate within sediments, sediment ero- derived authigenic carbonates are a by-product pelagic sediments and increases their resistance sion that is specifi cally focused at the seepage of the anaerobic oxidation of methane (AOM; to mechanical erosion. Without the increased

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A 1673 1658.5 1644 Water Depth (m)

′ ′ L M

L

M

1670 1615 1560 Water Depth (m)

1700

1798 1669 1540 B Water Depth (m)

VE ~ 3.5 x 20 m 100 m

Blanking

Blanking

LML′ M′

Figure 14. Map showing part of the multibeam data (color scale ranging from 1798 m to 1540 m water depths) collected during two autonomous underwater vehicle (AUV) dives surveying a section of the transform fault on the northeastern side of Guaymas Basin (see Fig. 1 for location). Patches of the distinctive rough topography are outlined with thin black lines. Two areas with distinctive rough topography are indicated with red boxes and illustrated in more detail in insets. Location of Chirp profi le segments L–L′ and M–M′ shown below are indicated on map. Chirp profi les resolve refl ectors to subbottom depths of up to 50 m in the basins. Profi les L–L′ and M–M′ show that the sediment layers thin and/or pinch out on the fl anks of the ridges (blue arrow) where the distinctive rough topography emerges. Seismic blanking zones underlie the areas of distinctive rough topography. Red arrow indicates location of Pinkie’s Vent (Paull et al., 2007). Depressions in A and B are indicated with green arrows. VE—vertical exaggeration.

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1340 1321.5 1303 Water Depth (m)

1600

1700

1400

1750 1500 1250 Water Depth (m)

Figure 15. Map showing part of the multibeam data (color scale ranging from 1750 m to 1250 m water depths) collected during two autono- mous underwater vehicle (AUV) dives that surveyed a section of the transform fault on the southwest side of Guaymas Basin (see Fig. 1 for location). Patches of the distinctive rough topography are outlined with thin black lines. Trough presumed to be associated with the trans- form fault is indicated with red arrow. Areas of distinctive rough seafl oor topography occur in the area shown in the red box in more detail.

strength provided by the authigenic carbonate Seismic Blanking Halos the presence of interstitial gas bubbles (e.g., Judd cement, the rugose bottom observed in the AUV and Hovland, 1992; Roberts, 1999), dispersed multibeam data and ROV images could not per- Chirp profi les characteristically show the gas hydrate (e.g., Lee and Dillon, 2001; Riedel sist. In areas experiencing net erosion, variations presence of layered sediments away from areas et al., 2006b), or authigenic carbonate (Riedel in cement content result in differential erosion of distinctive rough topography and exposed et al., 2002). Distinguishing between the poten- that leaves more densely cemented areas stand- authigenic carbonates (Figs. 3, 6, 7, 9, 10, 11, and tial causes of the blanking is not possible, and ing above the surrounding seafl oor. Authigeni- 13–15), but the lateral continuity of the refl ect- all three may be contributing to varying extents. cally cemented sediments are subject to brittle ing horizons is consistently lost within seismic However, the concentration of methane in pore failure (e.g., Bjørlykke and Hoeg, 1997), and blanking zones beneath the rough topography. waters that is needed to form either gas hydrate the presence of angular breccia (e.g., Bohrmann In some places, the loss of refl ectivity is because or gas bubbles is much higher than that needed et al., 1998) confi rms that brittle failures do take the layered sediments pinch out at the seafl oor to stimulate the formation of methane-derived place. Cement addition also reduces the sedi- (Figs. 9, 10, and 11); in other places, no change authigenic carbonate (e.g., Ussler and Paull, ment porosity and permeability, forms barriers in orientation is seen in the refl ecting horizons 2008; Bhatnagar et al., 2008). For the blanking that inhibit the upward migration of hydrocar- near the areas of rough topography and seismic in these Chirp profi les (Figs. 3, 6, 7, 13, and 15) bons, sharpens near-seafl oor pore-water chemi- blanking zones (Figs. 3, 6, 7, 13, and 15). to be primarily attributed to the presence of gas cal gradients, and allows accumulation of gas In the absence of physical disruptions of hydrate and/or gas bubbles requires that these underneath carbonate pavements. the beds, seismic blanking can be attributed to phases were present underneath the entire

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50 m

A B 50 m

C 50 m D 50 m

Figure 16. Perspective views of four topographic mounds surveyed with the Monterey Bay Aquarium Research Institute (MBARI) autonomous underwater vehicle (AUV): (A, C) Guaymas Basin (Fig. 14); (B) Santa Monica Basin (Paull et al., 2008), location indicated in Figure 1; and (D) the Pinnacle on southern Hydrate Ridge (Suess et al., 2001; Figs. 11 and 12C). Note that while they are similar in size, the amount of surface roughness varies. All four images have the same vertical exaggeration (2.5×).

blanking zone when the surveys were con- 1985; MacDonald et al., 1994). Several-centi- areas (e.g., Mackay, 1998). The amount of ducted. This would require ongoing fl uxes of meter-thick layers of pure hydrate have been uplift will be equivalent to the total thickness methane capable of maintaining relatively high observed exposed on the seafl oor at Spinnaker of the pure hydrate layers formed in the near pore-water methane concentrations (i.e., tens of Vent (Fig. 5G), Barkley Canyon (Chapman subsurface. millimoles) over widespread areas. In contrast, et al., 2004; Hester et al., 2007), Hydrate Ridge The smallest seafl oor infl ation features, or the microbial communities that stimulate the (Bohrmann et al., 1998; Suess et al., 2001), and mounds, observed in these surveys on steep formation of authigenic carbonate require con- in the Gulf of California (Paull et al., 2007). slopes at Bubbly Gulch and Pinkies Vent (Figs. siderably lower methane concentrations (i.e., Drilling at Bullseye Vent (Riedel et al., 2006a) 5A and 14) were less than 1 m high, i.e., too <1 mM) and thus a considerably lower methane and Hydrate Ridge (Carson et al., 1994; Tréhu small to be detected in the AUV surveys but fl ux. Moreover, because the carbonate cements et al., 2003) recovered lenses of pure hydrate easily seen within the fi eld of view of an ROV. remain after the methane supply stops, condi- near the seafl oor as well. Seafl oor mounds at Bubbly Gulch have surface tions appropriate for carbonate formation do not Development of subsurface layers of pure cracks consistent with infl ation of the seafl oor have to occur over more than a small fraction of gas hydrate requires that sediment be excluded (Fig. 3). Bubbly Gulch is also the only area the entire area at any point in time. The blank- during gas hydrate formation. Analysis of pres- where, despite the methane seepage, carbon- ing zones within these high-resolution profi les sure core samples confi rms the existence of ates were not observed exposed on the seafl oor. may primarily refl ect the integrated impact of signifi cant grain displacements (Holland et al., These mounds may be too young to have accu- carbonate formation associated with a modest 2008). This process may be similar to the for- mulated extensive deposits of slowly growing amount of methane seepage along active con- mation of segregated ice in permafrost, where authigenic carbonate, or, if present, erosion has duits that migrated around or through these the formation of ice lenses in the subsurface not exposed them yet on the seafl oor. seepage areas over protracted periods of time. commonly results in the expansion of the sedi- Similarly, small circular mounds also occur ment column by 50% or more (Murton et al., at both Barkley Canyon and Spinnaker Vent Deformation Associated with 2006). By analogy, development of lenses of (Figs. 5F, 7, and 9). In both of these areas, open Gas Hydrate Formation pure hydrate near the seafl oor will result in cracks and lenses of apparently pure gas hydrate surface blistering (MacDonald et al., 1994; are exposed on the sides of these mounds (Fig. Gas hydrate has been observed to occur within Hovland and Svensen, 2006; Vardaro et al., 5G). The seafl oor infl ation and deformation sediments as solid lenses that are up to 1 m in 2006; Paull et al., 2008; Serié et al., 2012), the associated with the formation of hydrate lenses thickness (e.g., Kvenvolden and McDonald, gas hydrate equivalent of pingos in permafrost explain the formation of these mounds.

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The detailed morphologic surveys show simi- mon et al., 2008). Without a sustained methane Roberts , 2001; Gay et al., 2007; Freire et al., lar-sized mounds occurring in different areas, fl ux, diffusion and mixing with overlying sea- 2011). However, physical models of how gas which are all interpreted to be caused by infl a- water will decrease the methane concentration venting results in the excavation of seafl oor to tion related to gas hydrate growth in the near- within the sediment below the level needed for form the observed depressions are still lacking. surface sediments. For example, mounds with methane hydrate stability, and the hydrate lenses The possibility that seafl oor depressions elevations more than 10 m high occur on the NE will start to dissolve (Brewer et al., 2003; Zhang within these gas-rich environments are gener- side of the Guaymas Basin (Figs. 14, 16A, and and Xu, 2003; Rehder et al., 2004; Lapham ated by eruptive events that eject fl uids and 16C), the Santa Monica Basin (Fig. 16B; Paull et al., 2014). solids into the water column has been suggested et al., 2008), and on southern Hydrate Ridge When gas hydrate lenses dissolve, sediment (e.g., Hovland, 1989; Prior et al., 1989; Hovland (Figs. 11, 12C, and 16D). Topographic features overburden that previously was supported by gas et al., 2002; Bangs et al., 2010). Near-seafl oor of comparable size off West Africa have been hydrate collapses, refi lling the void previously gas pockets and overpressured conditions may attributed to seafl oor infl ation as well (Serié occupied by gas hydrate. However, irreversible occur when impermeable lenses of gas hydrate et al., 2012). changes associated with cement addition, inter- trap gas in the near subsurface (Flemings et al., While the general shapes of these features are nal deformation, and brittle failures prevent the 2003; Hornbach et al., 2004). However, the similar (Fig. 16), the extent of seafl oor rough- overburden from simply collapsing back into its dynamics of the proposed eruptive events are ness on their fl anks can be explained by varia- original position. This process contributes to the unclear, and questions remain regarding the tions in the amount of erosion. This is consistent formation of the distinctive topography that sur- required pressures and gas volumes needed to with what is known about the relative ages of rounds seafl oor methane seepage areas. generate such eruptions. these features. The Santa Monica mound (Fig. The distinctive rough surface morphologies A physical model has been proposed to 16B) is a late Holocene feature (Paull et al., revealed in these surveys have some morpho- explain how capillary forces in the pore spaces 2008) and has relatively smooth fl anks, while logic similarities to subaerial thermokarst land- within gas chimneys build slowly with time the Pinnacle on southern Hydrate Ridge (Fig. scapes (Washburn, 1980; Williams and Smith, until they abruptly fail, liquefying sediment and 16D), of early Holocene age or older (Suess 1989). Thermokarst landscape evolves by both erupting gas and presumably some liquefi ed et al., 2001; Teichert et al., 2003), exhibits a formation and decomposition of ice. Expan- sediment (Cathles et al., 2010). This model pre- much more eroded topography. sion during ice formation forces sediment apart dicts physical distortion of the underlying sedi- Still larger topographic highs, which are and results in surface infl ation and ice wedging, ment column, which is observed at some gas either circular or ovoid, occur at Eel Slump which break up rigid surfaces into slabs. Some chimneys elsewhere, but which is not observed (Fig. 13) and northern Hydrate Ridge (Figs. 10 of the textures associated with this deformation at these sites. Moreover, none of these seepage and 12A). As the topographic highs increase in remain imprinted in the sediments after the ice areas shows a recognizable halo of debris, as size, the extent to which infl ation is the cause thaws. This analogy may provide some explana- indicated by the eruptive model. These mor- for the increase in topography becomes more tion as to the way in which the distinctive rough phologies may be the result of the cumulative uncertain. Nevertheless, the >2-km-long ridge topography develops in areas that have experi- effects of less dramatic but observable processes on northern Hydrate Ridge has extensive open enced gas hydrate formation and decomposition. that occur slowly. depressions or cracks that suggest expansion Rafting away of pieces of gas hydrate (Figs. 10 and 12A). Seafl oor Erosion along with some adhering sediments has been observed at seepage sites (Paull et al., 1995, Hydrate Dissolution and Seafl oor Collapse The distinctive rough topography of the sea- 1999, 2002; Suess et al., 2001). When clumps fl oor suggests that hemipelagic sediment was of gas hydrate and adhering sediment occupy While the seafl oor in all the surveys presented exhumed, thereby exposing carbonates on the >83% of the volume, the sediment clump will here is at pressures and temperatures appropri- seafl oor, widening cracks, undercutting the be buoyant (given densities of 0.91 g/cm3, 1.03 ate for methane hydrate stability, pore waters edges between carbonate blocks, and produc- g/cm3, and 1.6 g/cm3, for hydrate, seawater, must have adequate concentrations of methane ing the depressions. Clearly, these are areas that and water-saturated clays, respectively). Once for methane hydrate to form (Zhang and Xu, have experienced erosion. Various mechanisms hydrate pieces break free from the bottom, the 2003; Bhatnagar et al., 2008; Sloan and Koh, to explain how fl uid seepage erodes the seafl oor rate of hydrate dissolution is comparatively slow 2008), yet methane concentrations in normal should be considered. relative to the rate of lateral transport in bottom seawater are signifi cantly lower than this. Thus, Patches of distinctive rough topography char- currents (Brewer et al., 2003); thus, the adhering methane hydrate occurrence on or very near acteristically occur on the surfaces of plateaus sediments may be moved a considerable distance the seafl oor requires a sustained advective fl ux and crests of ridges, sites that are preferentially before falling back to the seafl oor. Thus, sedi- of methane (Torres et al., 2004). Conversely, exposed to winnowing by bottom currents. ment excavation by hydrate fl otation may not when methane hydrate is exposed to seawater The rates of current-induced erosion decrease leave an identifi able halo of debris surrounding or pore waters with low methane concentration, signifi cantly as deep-sea sediments become the seepage area. These rafts can be small pieces it dissolves (Brewer et al., 2003; Zhang and Xu, cemented by carbonates (Young and Southard, of hydrate forming under an overhang, grow- 2003; Rehder et al., 2004; Lapham et al., 2014). 1978). Thus, in areas experiencing net erosion, ing in size, and breaking off periodically. The Over time, the subsurface plumbing associ- the more cemented patches stand elevated com- occurrence of larger rafts has also been inferred ated with seafl oor vents changes as a conse- pared to the adjacent less-cemented areas. (Bangs et al., 2010). quence of authigenic carbonate and gas hydrate Within the patches of distinctive rough topog- The erosional impact of the greatly enhanced formation, which alter the sediment porosity raphy, there are numerous small depressions biological activity supported by hydrocarbon and permeability and cause internal deformation (Figs. 4, 7, 10, 12A, 15, and 16). Similar depres- seepage (Sibuet and Olu, 1998; van Dover et al., within the sediment. Shifts in fl uid pathways sions occur at other active gas venting sites (e.g., 2002; Levin, 2005) is unquantifi ed. Bioerosion will result in some conduits being cut off (Solo- Hovland and Judd, 1988; Paull et al., 1995; is a signifi cant process in the reduction of hard

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bottom substrates in shallow water (e.g., Neu- (Ussler and Paull, 2008), the time needed to ered sediment stopped. The depth of the Bulls- mann, 1966; Glynn, 1997), especially where generate the large patches of distinctive rough eye Vent depression only requires an average high abundances of organisms and hard bot- topography may require thousands if not tens of removal rate of 0.4 mm of sediment per year, tom substrates occur. By analogy, it should thousands of years. which is a rather modest rate. Similar rates of be a factor in seep settings that also have high The age of sediment with known stratigraphic excavation applied over longer time intervals abundances of organisms and hard bottom sub- relationships to the patches of distinctive rough may be adequate to generate the more extensive strates. Abundant snails, urchins, crabs, and topography helps constrain the time required areas of distinctive rough topography observed other grazing organisms (Fig. 5H) rasp and for these morphologies to develop. The extreme elsewhere. scrape substrate while feeding on microbial example of this distinctive rough topography communities (Sibuet and Olu, 1998; van Dover occurs on Northern Hydrate Ridge, where some Progressive Maturity of Seepage-Related et al., 2002; Levin, 2005). The burrowing of local topographic highs have ~15 m of relief. Geomorphologies Vesicomya clams directly erodes fi rm substrates A 20-m-deep, ~100-m-wide, and 350-m-long (Paull et al., 2005). Enhanced bioturbation by open crack was mapped on the crest of a Considerable variation exists in the extent infauna irrigates the sediment, enhancing chemi- transverse ridge (Figs. 10 and 12A). The open of seepage-related geomorphic modifica- cal dissolution, and physically mobilizes and cracks suggest both expansion of the seafl oor tion between the surveyed sites. These varia- excretes fi ne sediment, which in part becomes surface and removal of material. The >49,400 tions are interpreted to refl ect varying stages resuspended (Ziebis and Haese, 2005; Meadow 14C yr B.P. age of the underlying sediments and of development associated with exposure to et al., 2011). Microbial mats thrive on and exposures of carbonate on the surface dating methane-rich fl uids. The size of the patches of within seep carbonates (Caldwell et al., 2008), to 267.6 ka (Teichert et al., 2003) indicate that distinctive rough topography is consistent with and such endolithic communities inevitably the patches of distinctive rough topography on what is known about their exposure history. For contribute to substrate reduction (e.g., Golubic Northern Hydrate Ridge may have developed example, Bullseye Vent is among the young- et al., 1975; Edwards et al., 2003). The effects over tens of thousands of years. Similarly, the est datable features, and it is associated with a of bioerosion are preferentially focused where sediments within the upper 11 cm of the seafl oor relatively small area (Table 3). Hydrate Ridge fl uid seepage is active, often in the bottoms of on southern Hydrate Ridge are also approach- has the largest patches of this distinctive rough the cracks and around the base of the carbon- ing 14C depletion (Table 2), indicating that the topography, the greatest seafl oor relief, the most ate blocks where the chemosynthetic organisms younger sediments have been removed or were extensive blanking zones, and appears to have occur. Bioerosion is inferred to be a signifi cant never deposited. The considerable areas of the longest exposure history. The observations process contributing to progressive deepening distinctive rough topography on both crests of documented in the seven surveyed areas of dis- of cracks, the undercutting of carbonate blocks, Hydrate Ridge are examples of comparatively tinctive rough topography are interpreted to rep- and excavation of depressions. The integrated mature seepage morphologies. resent varying stages in the geomorphic evolu- effect of these processes will have major impact The best age control to assess the duration tion of the seafl oor in the presence of methane on the local seafl oor morphology if the seepage of exposure to seepage conditions comes from venting and gas hydrate formation within near- is persistent. the areas surrounding Bullseye Vent (Fig. 8; seafl oor sediments. Table 2). No sediment occurs either inside Time Required to Form the Distinctive or outside Bullseye Vent that is younger than CONCLUSIONS Rough Topography 13,100 14C yr B.P. The profound change from rapid deposition prior to 13,100 14C yr B.P. to AUV bathymetric surveys of the best-known Some constraint on the amount of time being sediment-starved subsequently (Fig. 8D) methane seepage sites on the Pacifi c margin required to generate the observed areas of is consistent with the glacial history of British of North America show these sites in unprece- distinctive rough topography is provided by Columbia (Porter and Swanson, 1998; Clague dented detail and reveal the distinctive rough geologic context. Sediment-hosted methane- and James, 2002). The ice sheets that extended topography that characterizes methane seeps. derived authigenic carbonate exposed on the to the shelf edge and calved into the open ocean This distinctive topography includes a progres- seafl oor within these areas of distinctive rough reached their maximum extent ca. 14,000 14C yr sion of seafl oor depressions that range from topography originally formed within hemi- B.P. but retreated rapidly and were gone from <1 m to over 100 m in depth, and topographic pelagic sediments. They grow when and where most of what is now southern Vancouver Island highs that exceed 15 m of vertical relief. The zones in the subsurface are subjected to focused by 13,100 14C yr B.P. (Dallimore et al., 2008). origin of this morphology is attributed to multi- diagenesis in the sulfate-methane interface. The Thus, Bullseye Vent depression is in compari- ple processes that are stimulated by methane exposure of these carbonates on the seafl oor son a young feature having developed within the seepage. The increased biological activity asso- inherently implies erosion. last 13,100 yr, since the rain of glacially deliv- ciated with chemosynthetic biological commu- Inevitably, the positions of the active seep- age conduits shift as carbonates form and gas hydrate grows and dissolves. These shifts will TABLE 3. AREA COVERED BY AUTONOMOUS UNDERWATER VEHICLE (AUV) SURVEYS AND AREA AND alter the position of the sulfate-methane inter- PERCENTAGE OF DISTINCTIVE ROUGH TOPOGRAPHY (DRT) IDENTIFIED WITHIN EACH SURVEY face and thus where authigenic carbonates are Area of entire survey Total DRT Percent of total area Survey area (m2) (m2) (%) forming at any particular time. The observed Neptune cable route 30,302,744 321,123 1.06 patches of rough topography refl ect an amal- Barkley Canyon 8,452,059 8416 0.10 gamation of carbonates formed as the position Northern Hydrate Ridge 12,650,194 1,876,487 14.83 Southern Hydrate Ridge 9,215,356 471,144 5.11 of the seepage conduits shifted over time. As Eel Slump 10,953,542 129,064 1.18 individual authigenic carbonate nodules can NE Guaymas Basin 15,469,043 208,758 1.35 take hundreds if not thousands of years to form SW Guaymas Basin 11,064,406 44,175 0.40

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nities enhances bioerosion rates. Formation of Fairbanks, Alaska, Alaska Sea Grant College Program, Gas Hydrates (ICGH 2008), paper 5691: Vancouver, University of Alaska Fairbanks, doi:10 .4027 /mhmta Canada. lenses of pure hydrate in near-subsurface sedi- .2008 .04 . Hornbach, M.J., Saffer, D.M., and Holbrook, S.W., 2004, ments causes expansion, which in turn results Carson, B., Seke, E., Paskeviich, V., and Holmes, M.L., Critically pressured free-gas reservoirs below gas in seafl oor blistering and generation of open 1994, Fluid expulsion sites on the Cascadia accretion- hydrate provinces: Nature, v. 427, p. 142–144, doi: 10 ary prism: Mapping diagenetic deposits with processed .1038 /nature02172 . cracks and raised topographic features. Buoy- GLORIA imagery: Journal of Geophysical Research, Hough, G., Green, J., Fish, P., Mills, A., and Moore, R., ant clumps of hydrate periodically break loose v. 99, p. 11,959–11,969, doi: 10 .1029 /94JB00120 . 2011, A geomorphological mapping approach for from the seafl oor, rafting adhering sediment Cathles, L.M., Su, Z., and Chen, D., 2010, The physics of assessment of seabed geohazards and risk: Marine gas chimney and pockmark formation, with implica- Geophysical Researches, v. 32, p. 151–162, doi: 10 away. The dissolution of hydrate lenses when a tions of seafl oor hazards and gas sequestration: Marine .1007 /s11001 -010 -9111 -z . sustained methane fl ux ceases leaves the over- and Petroleum Geology, v. 27, p. 82–91, doi:10 .1016 /j Hovland, M., 1989, The formation of pockmarks and their .marpetgeo.2009 .09 .010 . potential infl uence on offshore construction: Quarterly lying sediment unsupported and results in sedi- Chapman, R., Pohlman, J., Coffi n, R., Chanton, J., and Journal of Engineering Geology and Hydrogeology, ment collapse to refi ll voids. The variations in Lapham, L., 2004, Thermogenic gas hydrates in the v. 22, p. 131–138, doi: 10 .1144 /GSL .QJEG .1989 .022 the morphologies shown in these seven surveys northern Cascadia margin: Eos, Transactions, Ameri- .02 .04 . can Geophysical Union, v. 85, p. 361–368, doi:10 .1029 Hovland, M., and Judd, A.G., 1988, Seabed Pockmarks and collectively provide a mosaic of observations /2004EO380001 . Seepages: Impact on Geology, Biology and the Marine that illustrate the increasing geomorphologic Chiocci, F.L., Cattaneo, A., and Urgeles, R., 2011, Seafl oor Environment: London, Graham & Trotman Ltd., 293 p. maturity associated with increasing duration of mapping for geohazard assessment: State of the art: Hovland, M., and Svensen, H., 2006, Submarine pingos: Marine Geophysical Researches, v. 32, p. 1–11, doi:10 Indicators of shallow gas hydrates in a pockmark at exposure to methane seepage. .1007 /s11001 -011 -9139 -8 . Nyegga, Norwegian Sea: Marine Geology, v. 228, Clague, J.J., and James, T.S., 2002, History and isostatic p. 15–23, doi: 10 .1016 /j .margeo .2005 .12 .005 . ACKNOWLEDGMENTS effects of the last ice sheet in southern British Colum- Hovland, M., Gardner, J.V., and Judd, A.G., 2002, The sig- bia: Quaternary Science Reviews, v. 21, p. 71–87, doi: nifi cance of pockmarks to understanding fl uid fl ow 10 .1016 /S0277 -3791 (01)00070 -1 . processes and geohazards: Geofl uids, v. 2, p. 127–136, The David and Lucile Packard Foundation pro- Clague, J.J., and Ward, B., 2011, Pleistocene glaciation of doi: 10 .1046 /j .1468 -8123 .2002 .00028 .x . vided support. Special thanks are given to the crews British Columbia: Developments in Quaternary Sci- Hughen, K.A., Lehman, S.J., Southon, J., Overpeck, J.T., of the R/V Zephyr, R/V Western Flyer, the Monterey ences, v. 15, p. 563–573, doi: 10 .1016 /B978 -0 -444 Marchal, O., Herring, C., and Turnbull, J., 2004, 14C Bay Aquarium Research Institute (MBARI) autono- -53447 -7 .00044 -1 . activity and global carbon cycle changes over the past mous underwater vehicle (AUV) group, and the Dallimore, A., Enkin, R.J., Pienitz, R., Southon, J.R., 50,000 years: Science, v. 303, p. 202–207, doi: 10 .1126 MBARI remotely operated vehicle pilots. Reviews Baker, J., Wright, C.A., Pedersen, T.F., Calvert, S.E., /science.1090300 . from Tom Lorenson, Pete Dartnell, and two anony- Ivanochko , T.S., and Thomson, R.E., 2008, Postglacial Hughes Clarke, J.E., Gardner, J.V., Torresan, M., and Mayer, mous reviewers greatly improved the manuscript. evolution of a Pacifi c coastal fjord in British Colum- L., 1998, The limits of spatial resolution achievable bia, Canada; interactions of sea-level change, crustal using a 30 kHz multibeam sonar: Model predictions response, and environmental fl uctuations; results from and fi eld results, in IEEE Oceans 98 Proceedings, v. 3, REFERENCES CITED MONA Core MD02–2494: Canadian Journal of Earth p. 1823–1827. Sciences, v. 45, p. 1345–1362, doi: 10 .1139 /E08 -042 . Hyndman, R.D., and Davis, E.E., 1992, A mechanism for Bangs, N.L., Hornback, M.J., Moore, G.F., and Park, J.-O., Edwards, K.J., Bach, W., and Rogers, D.R., 2003, Geo micro- the formation of methane hydrate and seafl oor bottom- 2010, Massive methane release triggered by seafl oor biology of ocean crust: A role for chemoautotrophic simulating refl ectors by vertical fl uid expulsion: Jour- erosion offshore southwestern Japan: Geology, v. 38, Fe-bacteria: The Biological Bulletin, v. 204, p. 180– nal of Geophysical Research, v. 97, p. 7025–7041, doi: p. 1019–1022, doi: 10 .1130 /G31491 .1 . 185, doi:10 .2307 /1543555 . 10 .1029 /91JB03061 . Bhatnagar, G., Chapman, W.G., Dickens, G.R., Dugan, B., Flemings, P.B., Liu, X., and Winters, W.J., 2003, Critical Hyndman, R.D., Spence, G.D., Chapman, N.R., Riedel, and Hirasaki, G.J., 2008, Sulfate–methane transition pressures and multiphase fl ow in the Blake Ridge gas M., and Edwards, R.N., 2001, Geophysical studies of as a proxy for average methane hydrate saturation hydrates: Geology, v. 31, p. 1057–1060, doi:10 .1130 marine gas hydrate in northern Cascadia, in Paull, C.K., in marine sediments: Geophysical Research Letters, /G19863 .1 . and Dillon, W.P., eds., Natural Gas Hydrates: Occur- v. 35, p. L03611, doi: 10 .1029 /2007GL032500 . Freire, M.A.F., Matsumoto, R., and Santos, L.A., 2011, rence, Distribution and Detection: American Geophysi- Bjørlykke, K., and Hoeg, K., 1997, Effects of burial dia- Structural-stratigraphic control on the Umitaka Spur cal Union Geophysical Monograph 124, p. 273–295. genesis on stresses, compaction and fl uid fl ow in sedi- gas hydrates of Joetsu Basin in the eastern margin Jones, A.T., Greinert, J., Bowden, D., Klaucke, I., Petersen, mentary basins: Marine and Petroleum Geology, v. 14, of Japan Sea: Marine and Petroleum Geology, v. 28, J., Netzeband, G., and Weinrebe, W., 2010, Acoustic p. 267–276, doi: 10 .1016 /S0264 -8172 (96)00051 -7 . p. 1967–1978, doi: 10 .1016 /j .marpetgeo.2010 .10 .004 . and visual characterization of methane-rich seabed Bohrmann, G., Greinert, J., Suess, E., and Torres, M.E., Gardner, J.V., Malik, M., and Walker, S., 2009, Plume 1400 seeps at Omakere Ridge on the Hikurangi margin, New 1998, Authigenic carbonates from the Cascadia sub- meters high discovered at the seafl oor off the north- Zealand: Marine Geology, v. 272, p. 154–169, doi: 10 duction zone and their relation to gas hydrate stability: ern California margin: Eos, Transactions, American .1016 /j .margeo .2009 .03 .008 . Geology, v. 26, p. 647–650, doi:10 .1130 /0091 -7613 Geophysical Union, v. 90, no. 32, p. 275, doi: 10 .1029 Judd, A.G., and Hovland, M., 1992, The evidence of shal- (1998)026 <0647: ACFTCS>2 .3 .CO;2 . /2009EO320003 . low gas in marine sediments: Continental Shelf Borowski, W.S., Paull, C.K., and Ussler, W., III, 1999, Global Gay, A., Lopez, M., Berndt, C., and Séranne, M., 2007, Research, v. 12, p. 1081–1095, doi:10 .1016 /0278 -4343 and local variations of interstitial sulfate gradients in Geological controls on focused fl uid fl ow associated (92)90070 -Z . deep-water, continental margin sediments: Sensitivity with seafl oor seeps in the Lower Congo Basin: Marine Kienast, S.S., and McKay, J.L., 2001, Sea surface tempera- to underlying gas hydrates: Marine Geology, v. 159, Geology, v. 244, p. 68–92, doi:10 .1016 /j .margeo .2007 tures in the subarctic northeast Pacifi c refl ect millen- p. 131–154, doi:10 .1016 /S0025 -3227 (99)00004 -3 . .06 .003 . nial-scale climate oscillations during the last 16 krys: Brewer, P.G., Paull, C.K., Peltzer, E.T., Ussler, W., III, Glynn, P.W., 1997, Bioerosion and coral reef growth: A Geophysical Research Letters, v. 28, p. 1563–1566, Rehder, G., and Friederich, G., 2003, Measurement dynamic balance, in Birkeland, C., ed., Life and Death doi: 10 .1029 /2000GL012543 . of the fate of gas hydrates during transit through the of Coral Reefs: New York, Chapman & Hall, p. 68–95. Kovanen, D.J., and Easterbrook, D.J., 2002, Paleodeviations ocean water column: Geophysical Research Letters, Golubic, S., Perkins, R.E., and Lucas, K.J., 1975, Boring of radiocarbon marine reservoir values for the north- v. 29, p. 198–203. microorganisms and microborings in carbonate sub- east Pacifi c: Geology, v. 30, p. 243–246, doi: 10 .1130 Caldwell, S.L., Laidler, J.R., Brewer, E.A., Everly, J.O., strates, in Frey, R.W., ed., The Study of Trace Fossils: /0091-7613 (2002)030 <0243: PORMRV>2 .0 .CO;2 . Sandborgh, S.C., and Colwell, F.S., 2008, Anaerobic New York, Springer, p. 229–259. Kulm, L.D., and Suess, E., 1990, Relationship between car- oxidation of methane: Mechanisms, bioenergetics, and Gwiazda, R., Paull, C.K., Caress, D.W., Lundsten, E., bonate deposits and fl uid venting: Oregon accretionary ecology of associated microorganisms: Environmental Anderson, K., and Thomas, H., 2011, Seafl oor mor- prism: Journal of Geophysical Research, v. 95, no. B6, Science & Technology, v. 42, p. 6791–6799, doi:10 phology associated with deep-water gas plumes near p. 8899–8915, doi: 10 .1029 /JB095iB06p08899 . .1021 /es800120b . Eel Canyon: San Francisco, California, American Geo- Kulm, L.D., Suess, E., Moore, J.C., Carson, B., Lewis, B.T., Caress, D.W., and Chayes, D.N., 1996, Improved process- physical Union, Fall Meeting, abstract OS13B–1529. Ritger, S.D., Kadko, D.C., Thornburg, T.M., Embley, ing of Hydrosweep DS multibeam data on the R/V Hester, K.C., Dunk, R.M., Walz, P.M., Peltzer, E.T., Sloan, R.W., Rugh, W.D., Massoth, G.J., Langseth, M.G., Maurice Ewing: Marine Geophysical Researches, E.D., and Brewer, P.G., 2007, Direct measurement of Cochrane, G.R., and Scamman, R.L., 1986, Oregon v. 18, p. 631–650, doi: 10 .1007 /BF00313878 . multi-component hydrates on the seafl oor: Pathways subduction zone: Venting, fauna and carbonates: Sci- Caress, D.W., Thomas, H., Kirkwood, W.J., McEwen, R., to growth: Fluid Phase Equilibria, v. 261, p. 396–406, ence, v. 231, p. 561–566, doi:10 .1126 /science .231 Henthorn, R., Clague, D.A., Paull, C.K., Paduan, J., doi: 10 .1016 /j .fl uid .2007 .07 .053 . .4738 .561 . and Maier, K.L., 2008, High-resolution multibeam, Holland, M., Schultheiss, P., Roberts, J., and Druce, M., Kvenvolden, K.A., 1999, Potential effects of gas hydrate on sidescan, and subbottom surveys using the MBARI 2008, Observed gas hydrate morphologies in marine human welfare: Proceedings of the National Academy AUV D. Allan B., in Reynolds, J.R., and Greene, H.G., sediments, in Englezos, P., and Ripmeester, J., eds., of Sciences of the United States of America, v. 96, eds., Marine Habitat Mapping Technology for Alaska: Proceedings of the 6th International Conference on p. 3420–3426, doi: 10 .1073 /pnas .96 .7 .3420 .

Geosphere, April 2015 511

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/491/3337001/491.pdf by guest on 25 September 2021 Paull et al.

Kvenvolden, K.A., and McDonald, T.J., 1985, Gas hydrates Paull, C.K., Ussler, W., III, and Borowski, W.S., 1999, Ori- Ritger, S., Carson, B., and Suess, E., 1987, Methane-derived of the Middle America Trench, Deep Sea Drilling Proj- gin of submarine pockmarks: Geo-Marine Letters, authigenic carbonates formed by subduction induced ect Leg 84: Initial Reports of the Deep Sea Drilling v. 19, p. 164–168, doi: 10 .1007 /s003670050104 . pore-water expulsion along the Oregon/Washington Project, v. 84, p. 667–682. Paull, C.K., Brewer, P.G., Ussler, W., III, and Peltzer, E.T., margin: Geological Society of America Bulletin, v. 98, Lapham, L.L., Wilson, R.M., McDonald, I.R., and Chanton, 2002, An experiment demonstrating that marine p. 147–156, doi: 10 .1130 /0016 -7606 (1987)98 <147: J.P., 2014, Gas hydrate dissolution rates quantifi ed with slumping is a mechanism to transfer methane from MACFBS>2 .0 .CO;2 . laboratory and seafl oor experiments: Geochimica et seafl oor gas hydrate deposits into the upper ocean and Roberts, H.H., 1999, Acoustic wipe-out zones—A paradox Cosmochimica Acta, v. 125, p. 492–503, doi:10 .1016 atmosphere: Geo-Marine Letters, v. 22, p. 198–203, for interpreting seafl oor geologic/geotechnical char- /j .gca .2013 .10 .030 . doi: 10 .1007 /s00367 -002 -0113 -y . acteristics (An example from Garden Banks 161), in Lee, M.W., and Dillon, W.P., 2001, Amplitude blanking Paull, C.K., Ussler, W., III, Greene, H.G., Barry, J., and Offshore Technology Conference Proceedings, v. 31, related to the pore fi lling of gas hydrate in sediments: Keaten, R., 2005, Bioerosion by chemosynthetic bio- p. 591–602. Marine Geophysical Researches, v. 22, p. 101–109, logic communities on Holocene submarine slide scars: Roberts, H.H., 2001, Fluid and gas expulsion on the north- doi: 10 .1023 /A: 1010371308699 . Geo-Marine Letters, v. 25, p. 11–19, doi:10 .1007 ern Gulf of Mexico continental slope: Mud-prone to Levin, L., 2005, Ecology of cold seep sediments: Inter- /s00367-004 -0184 -z . mineral-prone responses, in Paull, C.K., and Dillon, actions of fauna with fl ow, chemistry and microbes: Paull, C.K., Ussler, W., III, Peltzer, E.T., Brewer, P., Keaten, W.P., eds., Natural Gas Hydrates: Occurrence, Distri- Oceanography and Marine Biology, Annual Review, R., Mitts, P., Nealon, J., Greinert, J., Herguera, J.C., bution, and Detection: American Geophysical Union v. 43, p. 1–46. and Perez, E., 2007, Authigenic carbon entombed in Geophysical Monograph 124, p. 145–164. Lonsdale, P.F., 1985, A transform continental margin methane-soaked sediments from the northeastern Sakai, H., Gamo, T., Ogawa, Y., and Boulegue, J., 1992, rich in hydrocarbons, Gulf of California: American transform margin of the Guaymas Basin, Gulf of Cali- Stable isotopic ratios and origins of the carbonates Association of Petroleum Geologists Bulletin, v. 69, fornia: Deep-Sea Research, Part II, Topical Studies in associated with cold seepage at the eastern Nankai p. 1160–1180. Oceanography, v. 54, p. 1240–1267, doi:10 .1016 /j .dsr2 Trough: Earth and Planetary Science Letters, v. 109, MacDonald, I.R., Guinasso, N.L., Sassen, R., Brooks, .2007 .04 .009 . p. 391–404, doi: 10 .1016 /0012 -821X (92)90101 -Z . J.M., Lee, L., and Scott, K.T., 1994, Gas hydrate that Paull, C.K., Normark, W.R., Ussler, W., III, Caress, D.W., Serié, C., Huuse, M., and Schødt, N., 2012, Gas hydrate breaches the sea fl oor on the continental slope of the and Keaten, R., 2008, Association among active defor- pingos: Deep seafl oor evidence of focused fl uid fl ow Gulf of Mexico: Geology, v. 22, p. 699–702, doi: mation, mound formation, and gas hydrate growth and on the continental margins: Geology, v. 40, p. 207– 10 .1130 /0091 -7613 (1994)022 <0699: GHTBTS>2 .3 accumulation with the seafl oor of the Santa Monica 210, doi: 10 .1130 /G32690 .1 . .CO;2 . Basin, offshore California: Marine Geology, v. 250, Sibuet, M., and Olu, K., 1998, Biogeography, biodiversity Mackay, J.R., 1998, Pingo growth and collapse, Tuktoyak- p. 258–275, doi: 10 .1016 /j .margeo .2008 .01 .011 . and fl uid dependence of deep-sea cold-seep communi- tuk Peninsula area, western Arctic coast, Canada: A Peckmann, J., Reimer, A., Luth, U., Luth, C., Hansen, ties at active and passive margins: Deep-Sea Research, long-term fi eld study: Géographie Physique et Quater- B.T., Heinicke, C., Hoefs, J., and Reitner, J., 2001, Part II, Topical Studies in Oceanography, v. 45, p. 517– naire (University of Montreal), v. 52, p. 311. Methane-derived carbonates and authigenic pyrite from 567, doi: 10 .1016 /S0967-0645 (97)00074 -X . Meadow, P.S., Meadows, A., and Murray, J.M.H., 2011, Bio- the northwestern Black Sea: Marine Geology, v. 177, Sloan, E.D., and Koh, C.A., 2008, Clathrate Hydrates of logical modifi ers of marine benthic seascapes: Their p. 129–150, doi: 10 .1016 /S0025-3227 (01)00128 -1 . Natural Gases (3rd ed.): Boca Raton, Florida, Taylor & role as ecosystem engineers: Geomorphology, v. 157– Pohlman, J.W., Canuel, E.A., Chapman, N.R., Spence, G.D., Francis/CRC Press, 720 p. 158, p. 31–48. Whiticar, M.J., and Coffi n, R.B., 2005, The origin of Solomon, E.A., Kastner, M., Jannasch, H., Robertson, Merewether, R., Olsson, M.S., and Lonsdale, P., 1985, thermogenic gas hydrates on the northern Cascadia G., and Weinstein, Y., 2008, Dynamic fl uid fl ow and Acoustically detected hydrocarbon plumes rising from margin as inferred from isotopic (13C/12C and D/H) chemical fl uxes associated with a seafl oor gas hydrate 2-km depths in Guaymas Basin, Gulf of California: and molecular composition of hydrate and vent gas: deposit on the northern Gulf of Mexico slope: Earth Journal of Geophysical Research, v. 90, p. 3075–3085, Organic Geochemistry, v. 36, p. 703–716, doi: 10 .1016 and Planetary Science Letters, v. 270, p. 95–105, doi: doi: 10 .1029 /JB090iB04p03075 . /j .orggeochem.2005 .01 .011 . 10 .1016 /j .epsl .2008 .03 .024 . Mix, A.C., Lund, D.C., Pisias, N.G., Bodén, P., Bornmalm, Porter, S.C., and Swanson, T.W., 1998, Radiocarbon age Southon, J.R., Nelson, D.E., and Vogel, J.S., 1990, A record L., Lyle, M., and Pike, J., 1999, Rapid climate oscil- constraints on rates of advance and retreat of the Puget of past ocean-atmosphere radiocarbon differences from lations in the northeast Pacifi c during the last degla- Lobe of the Cordilleran ice sheet during the last glacia- the Northeast Pacifi c: Paleoceanography, v. 5, p. 197– ciation refl ect Northern and Southern Hemisphere tion: Quaternary Research, v. 50, p. 205–213, doi:10 206, doi: 10 .1029 /PA005i002p00197 . sources, in Clark, P.U., Webb, R.S., and Keigwin, L.D., .1006 /qres .1998 .2004 . Spence, G.D., Chapman, N.R., Hyndman, R.D., and Cleary, eds., Mechanisms for Global Climate Change at Mil- Prior, D.B., Doyle, E.E., and Kaluza, M.J., 1989, Evidence C., 2001, Fishing trawler nets massive “catch” of meth- lennial Time Scales: American Geophysical Union for sediment eruption on deep sea fl oor: Gulf of Mex- ane hydrate: Eos, Transactions, American Geophysical Geophysical Monograph 112, p. 127–148. ico Science, v. 243, p. 517–519. Union, v. 82, p. 621, doi: 10 .1029 /01EO00358 . Murton, J.B., Peterson, R., and Ozouf, J.-C., 2006, Bedrock Rehder, G., Kirby, S.H., Durham, W.B., Stern, L.A., Peltzer, Stakes, D.S., Orange, D., Paduan, J., Salamy, K., and fracture by ice segregation in cold regions: Science, E.T., Pinkston, J., and Brewer, P.G., 2004, Dissolu- Maher, N., 1999, Origin of authigenic cold-seep car- v. 314, p. 1127–1129, doi: 10 .1126 /science .1132127 . tion rates of pure methane hydrate and carbon-dioxide bonates from Monterey Bay: Marine Geology, v. 159, Naudts, L., Greinert, J., Artemov, Y., Beaubien, S.E., hydrate in undersaturated seawater at 1000-m depth: p. 93–109, doi: 10 .1016 /S0025-3227 (98)00200 -X . Borowski, C., and De Batist, M., 2008, Anomalous Geochimica et Cosmochimica Acta, v. 68, p. 285–292, Stuiver, M., and Polach, H.A., 1977, Discussion: Reporting sea-floor backscatter patterns in methane venting doi: 10 .1016 /j .gca .2003 .07 .001 . of 14C data: Radiocarbon, v. 19, p. 355–363. areas, Dnepr paleo-delta, NW Black Sea: Marine Riedel, M., Hyndman, R.D., Spence, G.D., and Chapman, Stuiver, M., and Reimer, P.J., 1993, Extended 14C database Geology, v. 251, p. 253–267, doi: 10 .1016 /j .margeo N.R., 2002, Seismic investigations of a vent fi eld asso- and revised CALIB radiocarbon calibration program .2008.03 .002 . ciated with gas hydrates, offshore Vancouver Island: (version 5.0): Radiocarbon, v. 35, p. 215–230. Neumann, A.C., 1966, Observations on coastal erosion in Journal Geophysical Research, v. 107, no. B9, p. EPM Suess, E.M., Torres, M.E., Bohrmann, G., Collier, R.W., Bermuda and measurements of the boring rate of the 5-1–EPM 5-16. Greinter, J., Linke, P., Rehter, G., Tréhu, A.M., Wallmann, sponge Cliona lampa: Limnology and Oceanography, Riedel, M., Collett, T.S., and Hyndman, R.D., 2005, Gas K., Winckler, G., and Zulegger, E., 1999, Gas hydrate v. 11, p. 92–108, doi: 10 .4319 /lo .1966 .11 .1 .0092 . Hydrate Concentration Estimates from Chlorinity, destabilization: Enhanced dewatering, benthic mate- Novosel, I., Spence, G.D., and Hyndman, R.D., 2005, Electrical Resistivity, and Seismic Velocity: Geological rial turnover, and large methane plumes at the Cascadia Reduced magnetization produced by increased meth- Survey Canada Open-File Report 4934, 28 p. convergent margin: Earth and Planetary Science Letters, ane fl ux at a gas hydrate vent: Marine Geology, v. 216, Riedel, M., Willoughby, E.C., Chen, M.A., He, T., Novosel, v. 170, p. 1–15, doi:10 .1016 /S0012 -821X (99)00092 -8 . p. 265–274, doi: 10 .1016 /j .margeo .2005 .02 .027 . I., Schwalengerg, K., Hyndman, R.D., Spence, G.D., Suess, E., Torres, M.E., Bohrmann, G., Collier, R.W., Paull, C.K., and Ussler, W., III, 2008, Re-evaluation of the Chapman, N.R., and Edwards, R.N., 2006a, Gas Rickert, D., Goldfinger, C., Linke, P., Heuser, A., signifi cance of seafl oor accumulations of methane- hydrate on the northern Cascadia margin: Regional Sahling , H., Heeschen, K., Jung, C., Nakamura, derived carbonates: Seepage or erosion indicators?, geophysics and structural framework: Proceedings K., Greinert, J., Pfannkuche, O., Trehu, A., Klink- in Englezos, P., and Ripmeester, J., eds., Proceedings of the International Ocean Drilling Program, v. 311, hammer, G., Whiticar, M.J., Eisenhauer, A., Teichert, of the 6th International Conference on Gas Hydrates p. 1–28. B., and Elvert, M., 2001, Sea fl oor methane hydrates (ICGH 2008): Vancouver, British Columbia, Canada, Riedel, M., Novosel, I., Spence, G.D., Hyndman, R.D., at Hydrate Ridge, Cascadia margin, in Paull, C.K., and paper 5581. Chapman, N.R., and Lewis, T., 2006b, Geophysi- Dillon, W.P., eds., Natural Gas Hydrates: Occurrence, Paull, C.K., Chanton, J., Neumann, A.C., Coston, J.A., cal and geochemical signatures associated with gas Distribution and Detection: American Geophysical Martens, C.S., and Showers, W., 1992, Indicators hydrate–related venting in the northern Cascadia mar- Union Geophysical Monograph 124, p. 87–98. of methane-derived carbonates and chemosynthetic gin: Geological Society of America Bulletin, v. 118, Teichert, B.M.A., Eisenhauer, A., Bohrmann, G., Haase- organic carbon deposits: Examples from the Florida p. 23–38, doi: 10 .1130 /B25720 .1 . Schramm, A., Bock, B., and Linke, P., 2003, U/Th Escarpment: Palaios, v. 7, p. 361–375. Riedel, M., Trehu, A.M., and Spence G.S., 2010, Charac- systematics and ages of authigenic carbonates from Paull, C.K., Spiess, F.N., Ussler, W., III, and Borowski, terizing the thermal regime of cold vents at northern Hydrate Ridge, Cascadia margin: Recorders of fl uid W.A., 1995, Methane-rich plumes on the Carolina Cascadia margin from bottom-simulating refl ector dis- fl ow variations: Geochimica et Cosmochimica Acta, continental rise: Associations with gas hydrates: Geol- tributions, heat-probe measurements and borehole tem- v. 67, p. 3845–3857. ogy, v. 23, p. 89–92, doi:10 .1130 /0091 -7613 (1995)023 perature data: Marine Geophysical Researches, v. 31, Torres, M.E., Wallmann, K., Tréhu, A.M., Bohrmann, G., <0089: MRPOTC>2 .3 .CO;2 . p. 1–16, doi: 10 .1007 /s11001 -010 -9080 -2 . Borowski, W.S., and Tomaru, H., 2004, Gas hydrate

512 Geosphere, April 2015

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/491/3337001/491.pdf by guest on 25 September 2021 Distinctive rough seafl oor topography

growth, methane transport, and chloride enrichment at tion of gas hydrate beneath southern Hydrate Ridge: Westbrook, G.K., Carson, B., Musgrave, R.J., et al., the southern summit of Hydrate Ridge, Cascadia mar- Constraints from ODP Leg 204: Earth and Planetary Proceedings of the Ocean Drilling Program, Initial gin off Oregon: Earth and Planetary Science Letters, Science Letters, v. 222, p. 845–862, doi: 10 .1016 /j .epsl Reports, Volume 146: College Station, Texas, Ocean v. 226, p. 225–241, doi: 10 .1016 /j .epsl .2004 .07 .029 . .2004 .03 .035 . Drilling Program, p. 389–396, doi: 10 .2973 /odp .proc .ir Tréhu, A.M., Bohrmann, G., Rack, F., and Torres, M.E., Ussler, W., III, and Paull, C.K., 2008, Rates of anaerobic .146 -1 .012 .1994 . 2003, Leg 204 Scientifi c Party, in Tréhu, A.M., oxidation of methane and authigenic carbonate miner- Williams, P.J., and Smith, M.W., 1989, The Frozen Earth: Bohrmann, G., Rack, F., Torres, M.E., Bangs, N.L., alization in methane-rich deep-sea sediments inferred Fundamentals of Geocryology: Cambridge, UK, Cam- Barr, S.R., Borowski, W.S., Claypool, G.E., Collett, from models and geochemical profi les: Earth and Plan- bridge University Press, 306 p. T.S., Delwiche, M.E., Dickens, G.R., Goldberg, D.S., etary Science Letters, v. 266, p. 271–287, doi: 10 .1016 Young, R.N., and Southard, J.B., 1978, Erosion of fi ne- Gracia, E., Guerin, G., Holland, M., Johnson, J.E., /j .epsl.2007 .10 .056 . grained marine sediments: Sea-fl oor laboratory experi- Lee, Y-J, Liu, C-S., Long, P.E., Milkov, A.V., Riedel, van Dover, C.L., German, C.R., Speer, K.G., Parson, L.M., ments: Geological Society of America Bulletin, v. 89, M., Schultheiss, P., Su, X., Teichert, B., Tomaru, H., and Vrijenhoek, R.C., 2002, Evolution and the bio- p. 663–672, doi: 10 .1130 /0016 -7606 (1978)89 <663: Vanneste, M., Watanabe, M., and Weinberger, J.L., geography of deep-sea vent and seep invertebrates: EOFMSS>2 .0 .CO;2 . Proceedings of the Ocean Drilling Program, Initial Science, v. 295, p. 1253–1257, doi: 10 .1126 /science Zhang, Y., and Xu, Z., 2003, Kinetics of convective crystal Reports, Leg 204: College Station, Texas, Ocean Drill- .1067361 . dissolution and melting, with applications to methane ing Program, doi: 10 .2973 /odp .proc .ir .204 .2003 . Vardaro, M.F., MacDonald, I.R., Leslie, C., Bender, L.C., hydrate dissolution and dissociation in seawater: Earth Tréhu, A.M., Long, P.E., Torres, M.E., Bohrmann G., Rack, and Guinasso, N.L., Jr., 2006, Dynamic processes and Planetary Science Letters, v. 213, p. 133–148, doi: F.R., Collett, T.S., Goldberg, D.S., Milkov, A.V., observed at a gas hydrate outcropping on the continen- 10 .1016 /S0012 -821X (03)00297 -8 . Riedel, M., Schultheiss, P., Bangs, N.L., Barr, S.R., tal slope of the Gulf of Mexico: Geo-Marine Letters, Ziebis, W., and Haese, R.R., 2005, Fluid fl ow and biogeo- Borowski, W.S., Claypool, G.E., Delwiche, M.E., v. 26, p. 6–15, doi: 10 .1007 /s00367 -005 -0010 -2 . chemical interactions at methane seeps, in Kristensen, Dickens, G.R., Gracia, E., Guerin, G., Holland, M., Washburn, A.L., 1980, Geocryology: New York, Wiley & E., Haese, R.R., and Kostka, J.E., eds., Interactions Johnson, J.E., Lee, Y-J., Liu, C.-S., SU, X., Teichert, Sons, Halstead Press, 406 p. Between Macro- and Microorganisms in Marine B., Tomaru, H., Vanneste, M., Watanabe, M., and Westbrook, G.K., Carson, B., and Shipboard Scientific Sediments: Coastal and Estuarine Studies, v. 60, Weinberger, J.L., 2004, Three-dimensional distribu- Party, 1994, Summary of Cascadia drilling results, in p. 267–298.

Geosphere, April 2015 513

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